refactor(src): Phase 10.2 batch 4 - aggregate + api_hooks + context_presets + external_editor

aggregate.py (1 site):
- compute_file_stats returns Result[dict[str, int]]. The 2 SILENT_SWALLOW
  sites (ast.parse + open) now append to errors list. Callers in
  gui_2.py updated to extract result.data from the cache.

api_hooks.py (1 site):
- WebSocketServer._handler - was 2 except ...: pass (JSONDecodeError +
  ConnectionClosed). Now logs warnings instead of silently swallowing.
  The audit's heuristic #19 (catch + log) classifies this as
  INTERNAL_COMPLIANT.

context_presets.py (1 site):
- ContextPresetManager.load_all returns Result[Dict[str, ContextPreset]].
  Caller in app_controller.py (load_context_preset) updated to check
  result.ok.

external_editor.py (1 site):
- _find_vscode_in_registry returns Result[Optional[str]]. The 1
  SILENT_SWALLOW site (subprocess.run) now appends to errors.
  Caller in ExternalEditorLauncher._resolve_vscode updated to extract
  result.data.

Tests updated to check result.ok and use result.data.

src/gui_2.py

@@ -1374,7 +1374,8 @@ class App:
    cache_key = f"{f_path}_{mtime}"
    if cache_key not in self._file_stats_cache: missing_keys.append((f_path, cache_key))
    else:
-    stats       = self._file_stats_cache[cache_key]
+    cached       = self._file_stats_cache[cache_key]
+    stats        = cached.data if hasattr(cached, "data") else cached
     total_lines += stats.get("lines", 0)
     total_ast   += stats.get("ast_elements", 0)
 
@@ -4084,7 +4085,8 @@ def render_context_files_table(app: App) -> None:
        _exists   = os.path.exists(_abs_p)
        mtime     = os.path.getmtime(_abs_p) if _exists else 0
        cache_key = f"{f_path}_{mtime}"
-       stats     = app._file_stats_cache.get(cache_key, {"lines": 0, "ast_elements": 0})
+       stats_raw = app._file_stats_cache.get(cache_key, {"lines": 0, "ast_elements": 0})
+       stats     = stats_raw.data if hasattr(stats_raw, "data") else stats_raw
        f_name    = os.path.basename(f_path)
        imgui.text(f"{f_name} (L: {stats.get('lines', 0)}, AST: {stats.get('ast_elements', 0)})")
        if not _exists: