fix(mma_concurrent): import TrackMetadata directly to fix NameError

Root cause: src/app_controller.py:_start_track_logic_result used
'models.Metadata(...)' on line 4830 but the 'from src import models'
import was removed in commit ee763eea (the de-cruft migration).
The existing EXCEPT block catches only 7 exception types
(OSError, IOError, ValueError, TypeError, KeyError, AttributeError,
RuntimeError) - NOT NameError. So the NameError propagated up, the
io_pool worker died, and the for loop in _cb_accept_tracks._bg_task
never reached track-b.

Fix:
- Add TrackMetadata to the 'from src.mma import' line
- Change 'models.Metadata(...)' to 'TrackMetadata(...)'
- Restore the EXCEPT block to the original 7 types (narrowing the
  BaseException diagnostic back)

The diagnostic instrumentation logs are kept in this commit per
edit_workflow.md §9 ('diag lines are part of the same atomic commit
as the fix'). They will be removed in the Phase 2 cleanup commit.

Verified: test_mma_concurrent_tracks_execution now PASSES (35.88s
FAIL -> 7.95s PASS). Diag log shows full pipeline:
  _cb_accept_tracks -> _bg_task (2 tracks) -> Track A pipeline
  complete -> Track B pipeline complete -> 2 tracks in self.tracks.

src/app_controller.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 from src.tool_presets import ToolPreset
-from src.mma import Ticket, Track, TrackState
+from src.mma import Ticket, Track, TrackState, TrackMetadata
 from src.personas import Persona
 from src.mcp_client import MCPConfiguration, RAGConfig, load_mcp_config
 from src.project_files import ContextPreset, FileItem, NamedViewPreset, Preset
@@ -4796,8 +4796,16 @@ class AppController:
     print(f"Warning: No tickets generated for track: {title}")
     return OK
    self.ai_status = "Phase 2: Sorting tickets..."
+   try:
+    with open(b"C:\\projects\\manual_slop_tier2\\tests\\artifacts\\tier2_state\\fix_mma_concurrent_tracks_sim_20260627\\mma_diag.log", "ab") as _df:
+     _df.write(b"[DIAG] BEFORE _topological_sort_tickets_result\n")
+   except Exception: pass
    sort_result = self._topological_sort_tickets_result(raw_tickets, title)
    sorted_tickets_data = sort_result.data
+   try:
+    with open(b"C:\\projects\\manual_slop_tier2\\tests\\artifacts\\tier2_state\\fix_mma_concurrent_tracks_sim_20260627\\mma_diag.log", "ab") as _df:
+     _df.write(f"[DIAG] AFTER sort sorted_count={len(sorted_tickets_data) if sorted_tickets_data else 0} type={type(sorted_tickets_data[0]).__name__ if sorted_tickets_data else None}\n".encode())
+   except Exception: pass
    # 3. Create Track and Ticket objects (sorted_tickets_data is list[Ticket])
    tickets = []
    for t_data in sorted_tickets_data:
@@ -4814,10 +4822,18 @@ class AppController:
     tickets.append(ticket)
    track_id = f"track_{uuid.uuid5(uuid.NAMESPACE_DNS, f'{self.active_project_path}_{title}').hex[:12]}"
    track = Track(id=track_id, description=title, tickets=tickets)
+   try:
+    with open(b"C:\\projects\\manual_slop_tier2\\tests\\artifacts\\tier2_state\\fix_mma_concurrent_tracks_sim_20260627\\mma_diag.log", "ab") as _df:
+     _df.write(b"[DIAG] BEFORE save_track_state\n")
+   except Exception: pass
    # Initialize track state in the filesystem
-   meta = models.Metadata(id=track_id, name=title, status="todo", created_at=datetime.now(), updated_at=datetime.now())
+   meta = TrackMetadata(id=track_id, name=title, status="todo", created_at=datetime.now(), updated_at=datetime.now())
    state = TrackState(metadata=meta, discussion=[], tasks=tickets)
    project_manager.save_track_state(track_id, state, self.active_project_root)
+   try:
+    with open(b"C:\\projects\\manual_slop_tier2\\tests\\artifacts\\tier2_state\\fix_mma_concurrent_tracks_sim_20260627\\mma_diag.log", "ab") as _df:
+     _df.write(b"[DIAG] AFTER save_track_state\n")
+   except Exception: pass
    # Add to memory and notify UI
    self.tracks.append({"id": track_id, "title": title, "status": "todo"})
    try:
@@ -4850,12 +4866,6 @@ class AppController:
    sys.stderr.flush()
    return OK
   except (OSError, IOError, ValueError, TypeError, KeyError, AttributeError, RuntimeError) as e:
-   import traceback
-   try:
-    with open(b"C:\\projects\\manual_slop_tier2\\tests\\artifacts\\tier2_state\\fix_mma_concurrent_tracks_sim_20260627\\mma_diag.log", "ab") as _df:
-     _df.write(f"[DIAG] _start_track_logic_result EXCEPTION title={title!r} {type(e).__name__}: {e}\n".encode())
-     traceback.print_exc(file=_df)
-   except Exception: pass
    err = ErrorInfo(kind=ErrorKind.INTERNAL, message=str(e),
       source="app_controller._start_track_logic_result", original=e)
    return Result(data=None, errors=[err])