feat: branching read-side filter — event readers consult active branch range (T113)

Wire the active branch's [origin_event_id, head_event_id] window into
every user-facing event/memory reader so switching branches actually
changes what dialogue and memories the user sees. Phase 4 T89/T94
shipped branches as metadata-only — this closes the loop.

Helper:
- chat/state/branches.py: add `active_branch_event_ids(conn)` returning
  the active branch's id range, with two defensive fall-throughs to
  `(0, BIG_INT)`: (a) no active branch row at all, and (b) the
  bootstrap "main" sentinel (name="main", origin=0, head=0). Production
  never bumps main's head_event_id today, so this preserves existing
  reader behaviour for every test that doesn't explicitly switch.

Readers updated (all user-facing dialogue / retrieval surfaces):
- chat/services/turn_common.py::read_recent_dialogue — chat-history
  prompt context + the chat-view template path (via web/turns.py +
  web/chat.py).
- chat/services/scene_summarize.py::_read_recent_dialogue — scene-close
  per-POV summary input.
- chat/state/memory.py::search_memories — FTS leg filters via
  m.event_id (T109's column); legacy NULL event_id rows are *included*
  unconditionally so the filter doesn't break pre-0014 retrieval. The
  fused (FTS + RRF + vector) path also drops vector hits whose
  event_id falls outside the branch window.
- chat/web/meanwhile.py::_read_recent_meanwhile_dialogue — meanwhile
  prompt context.

Projector queries (chat/state/world.py et al.) and admin/management
surfaces (drawer hide-panel, cross-chat search, regenerate's row
lookups by id) are intentionally NOT branch-filtered: projection must
see the full log to build state correctly, and the admin surfaces
operate across branches by design.

Tests (10 new, 446 total):
- tests/test_branches_state.py: 3 tests for `active_branch_event_ids`
  itself (bootstrap-main, no-active-branch, non-main literal range).
- tests/test_branching.py: 7 cross-feature tests covering the spec's
  five required scenarios plus scene_summarize and meanwhile readers.

chat/services/scene_summarize.py

@@ -144,23 +144,36 @@ def _read_recent_dialogue(
     ``id >= since_event_id`` so callers needing a scene-scoped view (e.g.
     thread detection on close) don't pull turns that landed before the
     closing scene's ``scene_opened`` event.
+
+    T113: also clamps by the active branch's ``[origin, head]`` event-id
+    range so scene-summary inputs respect the user's current branch.
+    Bootstrap-main and "no active branch" fall through to ``(0, BIG_INT)``
+    so existing flows are unchanged.
     """
+    from chat.state.branches import active_branch_event_ids
+
+    origin, head = active_branch_event_ids(conn)
     if since_event_id is None:
         cur = conn.execute(
             "SELECT kind, payload_json FROM event_log "
             "WHERE kind IN ('user_turn', 'assistant_turn') "
             "  AND superseded_by IS NULL AND hidden = 0 "
+            "  AND id BETWEEN ? AND ? "
             "ORDER BY id DESC LIMIT ?",
-            (limit,),
+            (origin, head, limit),
         )
     else:
+        # Compose ``since_event_id`` with the branch lower bound — readers
+        # want the tightest ``id >= max(since, origin)`` clamp without an
+        # extra Python pass.
+        lower = max(origin, since_event_id)
         cur = conn.execute(
             "SELECT kind, payload_json FROM event_log "
             "WHERE kind IN ('user_turn', 'assistant_turn') "
             "  AND superseded_by IS NULL AND hidden = 0 "
-            "  AND id >= ? "
+            "  AND id BETWEEN ? AND ? "
             "ORDER BY id DESC LIMIT ?",
-            (since_event_id, limit),
+            (lower, head, limit),
         )
     rows = list(reversed(cur.fetchall()))
     out: list[dict] = []