feat: concurrent dispatch for fs/workspace WebSocket requests

[veithly]

Summary

• head-of-line blocking fix: fs/workspace RPC methods (fs.list, fs.stat, fs.read, fs.write, etc.) are now dispatched as concurrent asyncio.Tasks instead of being awaited serially in the message loop, eliminating request queuing when clients send multiple fs requests in quick succession
• per-path write locks: added _PathLockManager to WorkspaceFSService that serializes concurrent writes to the same path while allowing different paths to write in parallel; locks are acquired in sorted order to prevent deadlock on rename
• WebSocket send safety: added send_lock per Connection to serialize send_json calls and prevent frame interleaving from concurrent tasks

Changes

File	Change
handler.py	Added _CONCURRENT_METHODS frozenset; message loop now uses create_task for fs/workspace methods; _concurrent_tasks set tracks in-flight tasks for cleanup on disconnect
manager.py	Connection dataclass gains send_lock: asyncio.Lock; send_message wraps send_json with it
workspace_fs.py	New _PathLockManager class; write/mkdir/rename/remove wrapped with path locks; read methods (list/stat/read) unchanged
tests/test_workspace_fs_service.py	5 new concurrency tests

Test Plan

• test_concurrent_reads_run_in_parallel — 15 parallel read ops complete correctly
• test_concurrent_writes_same_path_serialized — same-path writes do not overlap (verified via timing)
• test_concurrent_writes_different_paths_parallel — different-path writes run in parallel (elapsed < 120ms for 3×50ms writes)
• test_rename_no_deadlock — rename with two-path lock completes within 2s timeout
• test_path_lock_cleanup — _PathLockManager cleans up lock entries after release