refactor(multiple): continue Phase 6 Optional[T] elimination (batch 3)

Phase 6: Eliminate Optional[T] returns - BATCH 3 of 7
Before: 4 more Optional[T] returns removed
After:  0 in app_controller.py (Pending MMA), project_manager.py
        (load_track_state), session_logger.py (log_tool_call),
        models.py (TrackState.metadata defaults)
Delta:  -4 sites (cumulative: -19 of 30)

Specific changes:
- src/app_controller.py:2781,2785: _pending_mma_spawn, _pending_mma_approval
  return Metadata() (zero-init sentinel) when no pending items
- src/project_manager.py:301: load_track_state returns EMPTY_TRACK_STATE
  sentinel (added to models.py) when no state file exists or load fails
- src/models.py:476: TrackState.metadata now has default_factory=dict;
  EMPTY_TRACK_STATE = TrackState() added as module-level sentinel
- src/session_logger.py:166: log_tool_call returns str (was Optional[str])

Test impact:
- test_track_state_persistence.py: 4 tests pass (existing tests)
- test_app_controller_result.py: 12 tests pass

Verification:
- audit_weak_types --strict: OK (107 <= 112 baseline)
- py_check_syntax: OK on all changed files
- 44 tests pass (test_track_state_persistence, test_track_state_schema,
  test_session_logger_optimization, test_app_controller_result)

REMAINING: ~11 Optional[T] returns in:
- src/external_editor.py (3 - get_editor, _find_vscode_common_paths,
  auto_detect_vscode)
- src/file_cache.py (7 - tree_sitter.Node walks + get_file_id)
- src/diff_viewer.py (1 - parse_hunk_header)

src/app_controller.py

@@ -2778,12 +2778,12 @@ class AppController:
    )])
 
  @property
- def _pending_mma_spawn(self) -> Optional[Metadata]:
-  return self._pending_mma_spawns[0] if self._pending_mma_spawns else None
+ def _pending_mma_spawn(self) -> Metadata:
+  return self._pending_mma_spawns[0] if self._pending_mma_spawns else Metadata()
 
  @property
- def _pending_mma_approval(self) -> Optional[Metadata]:
-  return self._pending_mma_approvals[0] if self._pending_mma_approvals else None
+ def _pending_mma_approval(self) -> Metadata:
+  return self._pending_mma_approvals[0] if self._pending_mma_approvals else Metadata()
 
  @property
  def current_provider(self) -> str:

src/models.py

@@ -474,7 +474,7 @@ class Metadata:
 
 @dataclass
 class TrackState:
- metadata:   Metadata
+ metadata:   Metadata      = field(default_factory=dict)
  discussion: List[str]    = field(default_factory=list)
  tasks:      List[Ticket] = field(default_factory=list)
 
@@ -524,6 +524,10 @@ class TrackState:
    tasks      = [Ticket.from_dict(t) for t in data.get("tasks", [])],
   )
 
+
+EMPTY_TRACK_STATE: TrackState = TrackState()
+
+
 @dataclass
 class FileItem:
  path:            str

src/project_manager.py

@@ -298,18 +298,19 @@ def save_track_state(track_id: str, state: 'TrackState', base_dir: Union[str, Pa
  data = clean_nones(state.to_dict())
  with open(state_file, "wb") as f: tomli_w.dump(data, f)
 
-def load_track_state(track_id: str, base_dir: Union[str, Path] = ".") -> Optional['TrackState']:
+def load_track_state(track_id: str, base_dir: Union[str, Path] = ".") -> "TrackState":
  """
  Loads a TrackState object from conductor/tracks/<track_id>/state.toml.
+ Returns empty TrackState (zero-init) if not found.
  [C: tests/test_track_state_persistence.py:test_track_state_persistence]
  """
- from src.models import TrackState
+ from src.models import TrackState, EMPTY_TRACK_STATE
  state_file = paths.get_track_state_dir(track_id, project_path=str(base_dir)) / 'state.toml'
- if not state_file.exists(): return None
+ if not state_file.exists(): return EMPTY_TRACK_STATE
  try:
   with open(state_file, "rb") as f: data = tomllib.load(f)
  except (OSError, tomllib.TOMLDecodeError):
-  return None
+  return EMPTY_TRACK_STATE
  return TrackState.from_dict(data)
 
 def load_track_history(track_id: str, base_dir: Union[str, Path] = ".") -> list[str]:

src/session_logger.py

@@ -163,7 +163,7 @@ def log_comms(entry: dict[str, Any]) -> Result[bool]:
  except (OSError, TypeError, ValueError) as e:
   return Result(data=False, errors=[ErrorInfo(kind=ErrorKind.INTERNAL, message=str(e), source="session_logger.log_comms", original=e)])
 
-def log_tool_call(script: str, result: str, script_path: Optional[str]) -> Optional[str]:
+def log_tool_call(script: str, result: str, script_path: Optional[str]) -> str:
  """
  Append a tool-call record to the toolcalls log and write the PS1 script to
  the session's scripts directory. Returns the path of the written script file.