Add classic FS OPFS backend (MEMFS + sync worker) and shared IDBFS tests

- libopfs.js: WASMFS path unchanged; without WASMFS, implement IDBFS-like OPFS mount on top of MEMFS, persisting via a worker and sync access handles; store mtime/mode metadata in .emscripten-opfs-timestamps; Safari createSyncAccessHandle compatibility; autoPersist and syncfs queue/reconcile; OPFS.quit on exit.
- Rename shared browser tests to test_idbfs_opfs_*.c; use -DOPFS vs IDBFS and AUTO_PERSIST for both backends.
- test_browser: wire IDBFS tests to new paths; add opfs_* test variants; include $removeRunDependency for idbfs fsync pre.js.

Author: caiiiycuk