Thread safety and cache integrity improvements

[sreckoskocilic]

Summary

• fs_cache.py rewrite — full lock hierarchy documented and enforced: Cache._lock → CacheItem._children_lock → CacheItem._attr_locks_lock → per-attr RLock. Generation counter detects invalidation during query() and retries. New mutate() and clear_attr() methods for atomic cache operations
• CachedIterator concurrency fix — split into _lock (protects items/counts) and _source_lock (serializes source advancement). remove()/append() use proper count clamping. get_next() re-scans after acquiring _source_lock to avoid missed items
• mother_fs.py — _remove_from_parent / _add_to_parent use cache.mutate() instead of get-then-modify pattern; _clear_parent_stat invalidates parent's stat and icon cache on add/remove; notify_file_added clears child cache before notification; iterdir always wraps in CachedIterator
• record_files.py — missing _m_files[url] = file_ assignment when file passes filter but isn't in _m_files yet
• Event system — add_callback / remove_callback / trigger protected by Lock; trigger snapshots callback list before iteration
• DirectoryPane._broadcast — iterates over list(self._listeners) snapshot to avoid mutation during iteration
• file_under_cursor context manager — thread-local storage replaces monkey-patched method; try/finally ensures cleanup
• icon_provider.py — Lock around suffix cache dict; .message → str(e) for FontError compatibility
• model/__init__.py — _MAX_VISITED cap, sourceModel() null guard, iterative parent traversal, shutdown() method (shared with critical crashes branch)
• model/diff.py — 5 assert statements replaced with raise RuntimeError for production safety
• qt/thread.py — Executor cleanup: removed bare except, added KeyError guard on timer cleanup, deleteLater() ensures Qt event processing
• fman/__init__.py — APPDATA env var check on Windows; Canceled extends Exception not KeyboardInterrupt; listener iteration snapshots
• fs.py — FileSystem docstring added; _file_changed_callbacks protected by lock with snapshot-then-iterate pattern; copy/move docstrings clarify they receive full URLs
• 380+ lines of new tests — test_fs_cache.py (14 tests), test_mother_fs.py (11 tests), test_fs.py (3 tests) covering cache concurrency, generation-based invalidation, CachedIterator thread safety, and event callback thread safety

Test plan

• Navigate directories rapidly — verify no cache corruption or stale entries
• Delete/add files while directory is loading — verify parent cache invalidation
• Run test suite: all 246+ tests pass
• Stress test: rapid directory switching with concurrent file operations