feat: regenerate rolls back lifecycle transitions on supersede (T114.3)

Closes the T83.4 gap: when ``regenerate_assistant_turn`` supersedes an
assistant_turn that already produced lifecycle transitions, it now
emits an ``event_status_reverted`` (T114.2) for each transition tagged
with ``triggered_by_assistant_turn_id == original_assistant_event_id``
(T114.1 back-reference) before the regenerated narrative is
reclassified.

Mapping from forward kind to ``prior_status`` lives in
``_PRIOR_STATUS_MAP``:
  - event_started   → planned
  - event_completed → active
  - event_cancelled → active (best-effort default; cancellation can fire
    from either planned or active, but detect_event_transitions only
    surfaces currently-active rows so 'active' is the realistic prior)

Backward compatibility: lifecycle rows authored before T114.1 lack the
back-reference field. Those are skipped (DEBUG log per row) and
collected into a legacy WARNING that preserves the T83.4
observability contract — operators still see un-rolled-back
transitions, just from older logs.

The classify-and-emit pass below the rollback now operates against an
events projection that has already been reverted, so re-firing
``event_started``/``event_completed``/``event_cancelled`` for the
regenerated narrative is safe — no double-emit of promotion artifacts.

Spec tests:
- ``test_regenerate_rolls_back_event_started_from_superseded_turn``
- ``test_regenerate_rolls_back_event_completed_to_active`` (also
  exercises the multi-rollback loop: a turn that fired both a start
  and a completion gets two event_status_reverted rows in id order,
  with active as the final projection — matching the per-row replay
  semantics of the projector)
- ``test_regenerate_skips_events_without_back_reference`` (pins the
  legacy compatibility path with both DEBUG and WARNING expectations)

chat/services/regenerate.py

@@ -95,6 +95,27 @@ from chat.web.render import render_turn_html
 _log = logging.getLogger(__name__)
 
 
+# T114.3: map a lifecycle-transition event kind to the events-table
+# status it implicitly transitioned *from*. Regenerate uses this to pick
+# the ``prior_status`` value for the ``event_status_reverted`` rollback
+# event so the projector sets the row back to where it was before the
+# superseded turn fired the transition.
+#
+# - ``event_started`` was emitted when the row was 'planned' → revert to
+#   'planned'.
+# - ``event_completed`` was emitted when the row was 'active' → revert
+#   to 'active'.
+# - ``event_cancelled`` could have fired from either 'planned' or
+#   'active'. Best-effort default: 'active'. The forward transitions
+#   below only fire detect_event_transitions for currently-active rows,
+#   so 'active' is the realistic prior in practice.
+_PRIOR_STATUS_MAP: dict[str, str] = {
+    "event_started": "planned",
+    "event_completed": "active",
+    "event_cancelled": "active",
+}
+
+
 async def regenerate_assistant_turn(
     conn: Connection,
     client,
@@ -115,17 +136,18 @@ async def regenerate_assistant_turn(
     cannot be found — the FastAPI route translates this to 404.
 
     .. note::
-       **Lifecycle-rollback limitation (T83.4, Phase 4 follow-up).**
+       **Lifecycle rollback (T114, Phase 4.5).**
        When the superseded turn already produced lifecycle transitions
        (``event_started`` / ``event_completed`` / ``event_cancelled``),
-       this function does NOT roll those rows back before re-running
-       ``detect_event_transitions`` against the regenerated text. A
-       regenerate-after-completion can therefore double-emit promotion
-       artifacts if the new text re-completes the same event. Phase 3.5
-       only documents the gap and emits a WARNING log naming the
-       affected event_log ids; the actual undo pass is invasive
-       (re-projection / inverse-handler dispatch) and is deferred to
-       Phase 4. See the ``# T83.4`` block below for the warning emit.
+       this function emits an ``event_status_reverted`` event for each
+       so the events row's status returns to its prior value before the
+       regenerated narrative is reclassified. Backward compatibility:
+       lifecycle events authored before T114.1 lack the
+       ``triggered_by_assistant_turn_id`` payload field; rollback skips
+       those (logged at DEBUG) so historic rows are not retroactively
+       reverted. A WARNING about un-rolled-back transitions is still
+       emitted when stragglers are found — the rollback handles the
+       common case while older logs continue to need manual review.
     """
     chat = get_chat(conn, chat_id)
     if chat is None:
@@ -158,20 +180,21 @@ async def regenerate_assistant_turn(
     original_assistant_payload = json.loads(row[0])
     original_user_turn_id = original_assistant_payload.get("user_turn_id")
 
-    # T83.4: scan for downstream lifecycle transitions emitted by the
-    # superseded turn — they're not being rolled back (see method
-    # docstring). Heuristic: any ``event_started`` / ``event_completed``
-    # / ``event_cancelled`` event_log row with id strictly greater than
-    # the original assistant_turn's id was emitted as part of (or after)
-    # that turn's processing. Lifecycle events don't carry ``chat_id``
-    # in their payload (their payload references an ``event_id`` FK to
-    # the ``events`` table, which holds chat_id), so we join through
-    # ``events`` to scope to this chat.
-    #
-    # A WARNING log surfaces the affected event ids so operators can
-    # spot double-emit cases until the Phase 4 rollback pass lands.
+    # T114.3: roll back lifecycle transitions emitted by the superseded
+    # turn. The scan uses the same id-greater-than-superseded-turn
+    # heuristic as the legacy T83.4 warning, joined to ``events`` for
+    # chat scoping (lifecycle events don't carry chat_id in their
+    # payload — they reference an ``event_id`` FK to the ``events``
+    # table, which holds chat_id). For each row whose payload carries
+    # ``triggered_by_assistant_turn_id == original_assistant_event_id``
+    # (T114.1 back-reference), emit an ``event_status_reverted`` event
+    # so the events-row status returns to the pre-transition value.
+    # Lifecycle rows authored before T114.1 lack the back-reference;
+    # those are skipped (DEBUG log) and a WARNING tracks their count so
+    # operators still see legacy stragglers — preserves the T83.4
+    # observability contract for un-rolled-back transitions.
     unrolled_lifecycle = conn.execute(
-        "SELECT el.id, el.kind FROM event_log AS el "
+        "SELECT el.id, el.kind, el.payload_json FROM event_log AS el "
         "JOIN events AS ev "
         "  ON ev.event_id = json_extract(el.payload_json, '$.event_id') "
         "WHERE el.kind IN ("
@@ -182,18 +205,73 @@ async def regenerate_assistant_turn(
         "ORDER BY el.id ASC",
         (chat_id, original_assistant_event_id),
     ).fetchall()
-    if unrolled_lifecycle:
-        # T90.2: phrased as "at-or-after turn <id>" rather than "from
-        # superseded turn" because regenerating an OLDER turn lists
-        # intervening-turn transitions that legitimately stand on their
-        # own — those weren't authored by the superseded turn itself.
+    rolled_back_ids: list[int] = []
+    skipped_no_backref: list[int] = []
+    for el_id, el_kind, el_payload_json in unrolled_lifecycle:
+        try:
+            lifecycle_payload = json.loads(el_payload_json)
+        except (TypeError, ValueError):
+            skipped_no_backref.append(el_id)
+            continue
+        triggered_by = lifecycle_payload.get("triggered_by_assistant_turn_id")
+        if triggered_by != original_assistant_event_id:
+            # Either a legacy row (no field) or a transition triggered
+            # by a *different* turn — leave it alone. DEBUG so the
+            # message is available under verbose logging without
+            # spamming the default WARNING channel.
+            _log.debug(
+                "regenerate_assistant_turn: skipping rollback for "
+                "lifecycle event_log id=%d (kind=%s) — no back-reference "
+                "or different turn (triggered_by=%r vs superseded=%d)",
+                el_id,
+                el_kind,
+                triggered_by,
+                original_assistant_event_id,
+            )
+            if triggered_by is None:
+                skipped_no_backref.append(el_id)
+            continue
+        prior_status = _PRIOR_STATUS_MAP.get(el_kind)
+        if prior_status is None:
+            # Defensive: the SQL filter already restricts to the three
+            # known kinds, but a future schema addition shouldn't crash
+            # the rollback path.
+            continue
+        target_event_id = lifecycle_payload.get("event_id")
+        if target_event_id is None:
+            continue
+        append_and_apply(
+            conn,
+            kind="event_status_reverted",
+            payload={
+                "event_id": target_event_id,
+                "prior_status": prior_status,
+            },
+        )
+        rolled_back_ids.append(el_id)
+    if rolled_back_ids:
+        _log.info(
+            "regenerate_assistant_turn: rolled back %d lifecycle "
+            "transition(s) triggered by superseded turn %s "
+            "(event_log ids: %s)",
+            len(rolled_back_ids),
+            original_assistant_event_id,
+            rolled_back_ids,
+        )
+    if skipped_no_backref:
+        # T83.4 (legacy) compatibility: still warn about stragglers
+        # without the back-reference so operators can spot pre-T114
+        # double-emit risks. Phrased as "at-or-after turn <id>" per
+        # T90.2 — older transitions may legitimately belong to other
+        # turns.
         _log.warning(
             "regenerate_assistant_turn: %d lifecycle transition(s) "
-            "at-or-after turn %s are NOT being rolled back (Phase 4 "
-            "follow-up). Affected event ids: %s",
-            len(unrolled_lifecycle),
+            "at-or-after turn %s are NOT being rolled back (no "
+            "triggered_by_assistant_turn_id back-reference). "
+            "Affected event ids: %s",
+            len(skipped_no_backref),
             original_assistant_event_id,
-            [r[0] for r in unrolled_lifecycle],
+            skipped_no_backref,
         )
 
     # 1a. Look up any sibling interjection beat in the same turn group
@@ -716,11 +794,13 @@ async def regenerate_assistant_turn(
     # runs inline after a completion so promotion artifacts land in the
     # same regenerate path.
     #
-    # T83.4 follow-up: when a regenerate replaces a turn that had
-    # already produced event transitions, those original transitions
-    # are NOT undone here (Phase 4 work). A WARNING log earlier in this
-    # function names the affected event_log ids — see the T83.4 block
-    # near the function entry.
+    # T114.3: original-turn transitions emitted before this regenerate
+    # ran were rolled back at the top of the function (see the
+    # ``# T114.3`` block) by appending ``event_status_reverted`` for
+    # each. The classify-and-emit pass below now operates against an
+    # ``events`` projection that has already been reverted, so it can
+    # safely re-fire transitions for the regenerated narrative without
+    # double-emitting promotion artifacts.
     new_active_events = list_active_events(conn, chat_id)
     if new_active_events:
         lifecycle_decision = await detect_event_transitions(