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.

tests/test_branches_state.py

@@ -7,7 +7,13 @@ from chat.db.migrate import apply_migrations
 from chat.eventlog.log import append_event
 from chat.eventlog.projector import project
 import chat.state.branches  # registers handlers
-from chat.state.branches import active_branch, get_branch, list_branches
+from chat.state.branches import (
+    _NO_HEAD_CLAMP,
+    active_branch,
+    active_branch_event_ids,
+    get_branch,
+    list_branches,
+)
 
 
 def test_main_branch_bootstrapped_by_migration(tmp_path):
@@ -174,3 +180,83 @@ def test_branch_switched_unknown_name_warns(tmp_path, caplog):
 
         # The unknown name was not inserted as a side effect.
         assert get_branch(conn, "does_not_exist") is None
+
+
+def test_active_branch_event_ids_bootstrap_main_returns_no_clamp(tmp_path):
+    """Bootstrap "main" (origin=0, head=0) reads as the no-clamp sentinel.
+
+    Migration 0013 seeds main with both event-id columns at 0; production
+    today never emits ``branch_head_updated`` for main, so head stays at 0
+    even as events accumulate. The helper treats this exact bootstrap
+    state as "all events visible" (lower bound 0, upper bound BIG_INT) so
+    every existing reader stays branch-agnostic until a non-main branch
+    becomes active.
+    """
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        origin, head = active_branch_event_ids(conn)
+        assert origin == 0
+        assert head == _NO_HEAD_CLAMP
+
+
+def test_active_branch_event_ids_no_active_branch_falls_through(tmp_path):
+    """No active branch row at all → defensive ``(0, BIG_INT)``.
+
+    A switch to an unknown branch leaves zero rows with ``is_active=1``;
+    ``active_branch`` returns None. The helper must still hand readers a
+    workable range (the full log) so the read pipeline doesn't crash on
+    an inconsistent metadata state.
+    """
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        # Switching to a nonexistent branch clears is_active flags
+        # without setting any other branch active.
+        append_event(
+            conn,
+            kind="branch_switched",
+            payload={"name": "does_not_exist"},
+        )
+        project(conn)
+        assert active_branch(conn) is None
+
+        origin, head = active_branch_event_ids(conn)
+        assert origin == 0
+        assert head == _NO_HEAD_CLAMP
+
+
+def test_active_branch_event_ids_returns_actual_range_for_non_main(tmp_path):
+    """Non-main branches return their literal ``(origin, head)`` window.
+
+    A branch created at origin=10 + bumped to head=20 must surface as
+    (10, 20) so readers' ``BETWEEN`` clamp scopes to that window.
+    """
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        append_event(
+            conn,
+            kind="branch_created",
+            payload={
+                "name": "experiment",
+                "origin_event_id": 10,
+                "head_event_id": 10,
+                "chat_id": "c1",
+            },
+        )
+        append_event(
+            conn,
+            kind="branch_head_updated",
+            payload={"name": "experiment", "head_event_id": 20},
+        )
+        append_event(
+            conn,
+            kind="branch_switched",
+            payload={"name": "experiment"},
+        )
+        project(conn)
+
+        origin, head = active_branch_event_ids(conn)
+        assert origin == 10
+        assert head == 20