Stop retried and deleted actions from corrupting story context and memories

Three fallout bugs from keeping the retried action row alive (906ba42),
plus two long-standing cursor bugs the same investigation turned up.

Retry context leak: the row being regenerated is still attached to the
adventure, so it was replayed as established story and the model wrote a
continuation of the attempt it was meant to replace — the story visibly
blended both takes. It leaked into four places, not one: history replay,
story-card trigger matching, in-scene NPC detection, and the memory-bank
similarity query. Adds a shared context.story_actions(exclude_action_id),
threaded through build_context and retrieve_memories.

Memory holdback: a memory could summarize the just-generated turn; retry
rewrites Action.text but memory_cursor has already advanced, so the memory
was never regenerated and went on describing narration no longer in the
story. settled_story_actions() holds the newest action back one turn —
only the last action is retryable, so that makes it unreachable. The
settled list is always a prefix, so cursors stay valid and nothing is
skipped. The run_post_turn clamp deliberately still uses the full count:
clamping to settled rewinds legacy adventures a step and double-covers an
action.

Cursor bookkeeping: memory_cursor is a position into story_actions() while
Memory.source_* are Action.index values, and the two diverge as soon as
anything is deleted. Deleting a middle action slid a never-summarized
action into the covered range, skipping it forever; and pruning a memory
left the actions it covered stranded behind the cursor. Adds
note_action_removed() (called before the delete in delete_action and
undo_turn) and a rewind in prune_dangling_memories. delete_action also
now prunes at all, which it never did.

Not addressed: editing an already-summarized action still leaves its
memory stale, and the cumulative story summary can't have one fact
un-mixed from it.

117 backend tests pass, including new test_memory_settling.py (12) and
two retry-context regression tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UeQVy5bEjLhfgWNc27Efet

backend/app/context/builder.py

@@ -74,10 +74,24 @@ def _history_text(action: models.Action) -> str:
     return text
 
 
-def _visible_npcs(adventure: models.Adventure, stat_schema: dict) -> dict[str, str]:
+def story_actions(
+    adventure: models.Adventure, exclude_action_id: int | None = None
+) -> list[models.Action]:
+    """The adventure's non-empty actions, as the model should see them.
+
+    `exclude_action_id` drops one action from the story — used by retry, where
+    the row being regenerated is still attached to the adventure (it holds the
+    variant history) but must not appear in the context assembled to replace it.
+    """
+    return [
+        a for a in adventure.actions
+        if a.text.strip() and (exclude_action_id is None or a.id != exclude_action_id)
+    ]
+
+
+def _visible_npcs(actions: list[models.Action], stat_schema: dict) -> dict[str, str]:
     """Defined NPCs whose trigger words appear in the recent story — the ones
     "in scene", so only their stats get injected. Maps npc id -> display name."""
-    actions = [a for a in adventure.actions if a.text.strip()]
     recent = SEPARATOR.join(a.text for a in actions[-6:]).lower()
     visible: dict[str, str] = {}
     for npc_key, ndef in (stat_schema.get("npcs") or {}).items():
@@ -107,10 +121,13 @@ def build_context(
     adventure: models.Adventure,
     settings: models.Settings,
     memory_bank: dict | None = None,
+    exclude_action_id: int | None = None,
 ) -> tuple[str, str, dict]:
     """Returns (system_text, story_text, context_report). `memory_bank` is the
-    result of memorybank.retrieve_memories (None when the bank is off)."""
+    result of memorybank.retrieve_memories (None when the bank is off);
+    `exclude_action_id` omits one action from the story (see story_actions)."""
     script_mem = _script_memory(adventure)
+    actions = story_actions(adventure, exclude_action_id)
 
     # ----- Always-included components -----
     system_sections: list[Section] = [Section("narrator", settings.narrator_prompt.strip())]
@@ -123,7 +140,7 @@ def build_context(
         if guide:
             system_sections.append(Section("world_state_guide", guide))
         block = worldstate.render_state_section(
-            adventure.world_state, stat_schema, _visible_npcs(adventure, stat_schema)
+            adventure.world_state, stat_schema, _visible_npcs(actions, stat_schema)
         )
         if block:
             system_sections.append(Section("world_state", block))
@@ -163,7 +180,6 @@ def build_context(
     available = max(256, settings.context_token_budget - reserved)
 
     # ----- Story cards: triggered by recent story text (the window history could fill) -----
-    actions = [a for a in adventure.actions if a.text.strip()]
     trigger_window = truncate_to_last_tokens(SEPARATOR.join(a.text for a in actions), available)
     triggered = _match_cards(adventure.story_cards, trigger_window)