backlot phase 0 review fixes: fail-closed gates, atomic checkpoint writes, event attribution hardening

- gate = manifest OR caller (stricter wins); unknown pipeline_type now
  raises instead of silently disabling enforcement; corrupt manifest logs
  and falls back; misleading diagnostic fixed
- write_checkpoint backfills pipeline_type from project.json marker so
  omitting the kwarg can't bypass gates
- checkpoint writes are atomic (temp + os.replace); history archiving is
  copy-based and best-effort (Windows open-file safe)
- manifest loads cached (load_pipeline_readonly); stage gate lookup moved
  to pipeline_loader.get_stage_human_approval_default; PROJECTS_DIR unified
  in lib/paths.py
- events: containment + root-normalization for explicit project dirs, no
  ghost-project mkdir, 0.0 cost preserved, nested-call depth tag,
  wrapper simplified
- documentary-montage edit-director gate footer (was missed); AGENT_GUIDE
  no longer claims edit/compose always auto-proceed

lib/pipeline_loader.py

@@ -21,11 +21,31 @@ SCHEMA_PATH = (
 )
 
 
+from functools import lru_cache
+
+
+@lru_cache(maxsize=1)
 def _load_manifest_schema() -> dict:
     with open(SCHEMA_PATH) as f:
         return json.load(f)
 
 
+@lru_cache(maxsize=64)
+def _load_pipeline_cached(name: str, defs_dir_key: str) -> dict[str, Any]:
+    """Cached manifest load. Treat the returned dict as READ-ONLY."""
+    return load_pipeline(name, Path(defs_dir_key) if defs_dir_key else None)
+
+
+def load_pipeline_readonly(name: str, defs_dir: Optional[Path] = None) -> dict[str, Any]:
+    """Load a manifest through a cache. The result MUST NOT be mutated.
+
+    Manifests are immutable within a run; hot paths (gate checks on every
+    checkpoint write, board state derivation) should use this instead of
+    re-parsing YAML + re-validating the schema each call.
+    """
+    return _load_pipeline_cached(name, str(defs_dir) if defs_dir else "")
+
+
 def load_pipeline(name: str, defs_dir: Optional[Path] = None) -> dict[str, Any]:
     """Load and validate a pipeline manifest by name.
 
@@ -150,6 +170,18 @@ def get_stage_skill(manifest: dict, stage_name: str) -> Optional[str]:
     return None
 
 
+def get_stage_human_approval_default(manifest: dict, stage_name: str) -> Optional[bool]:
+    """Whether a stage gates on human approval. None if the stage isn't declared.
+
+    This is the single lookup used by gate enforcement (lib/checkpoint.py)
+    and the Backlot board — keep them reading the same field the same way.
+    """
+    for stage in manifest["stages"]:
+        if stage["name"] == stage_name:
+            return bool(stage.get("human_approval_default", False))
+    return None
+
+
 def get_stage_review_focus(manifest: dict, stage_name: str) -> list[str]:
     """Get the review focus items for a stage."""
     for stage in manifest["stages"]: