feat(p2-shim): add opfs filesystem adapter

[eduardomourar]

Adds a first-party OPFS-backed filesystem adapter for preview2-shim's browser wasi:filesystem implementation, plus two non-JSPI improvement candidates (advisory locking, in-memory symlink support).

• OpfsFilesystemAdapter (src/browser/opfs-filesystem.ts) - loads a preopened OPFS directory into an in-memory tree once (loadOpfsCapability()), so every Descriptor op stays fully synchronous - no JSPI required. Mutations flush back to OPFS automatically (debounced to a microtask); flush()/dispose() force it explicitly.
• Symlinks survive a flush + reload via a hidden sidecar file (.__wasi_symlinks__.json) per root, since OPFS has no native symlink concept.
• Advisory locking (lockShared/lockExclusive/tryLock*/unlock) on BrowserFilesystemDescriptor, backed by a same-process reader/writer lock in InMemoryFilesystemAdapter. new OpfsFilesystemAdapter({ lockManager: navigator.locks }) opts into best-effort cross-tab coordination via the Web Locks API - off by default. lockManager takes a narrow BrowserLockManager interface rather than the global navigator directly, so it's easy to fake in tests.
• _addPreopenWithAdapter (src/browser/filesystem.ts) lets a host wire a custom adapter into the top-level wasi:filesystem/preopens singleton that transpiled components import statically.

Related to: #144

[vados-cosmonic]

This is awesome! Thanks for working on this -- most of my nits are more about testing and reserving some future optionality.

One more thing I'd really love to have is an E2E test in the browser! We have machinery and browser automation already set up, but for some reason we don't actually use it from any of the tests... I think you should be able to copy the setup in other projects (jco, primarily), but also happy to leave that as a follow up (we can make a new issue to track that separately, it would be very easy for anyone else to pick up), if you'd rather get this in as-is!

[eduardomourar]

Added e2e coverage using the harness in test/common.ts (startTestServer/runBasicHarnessPageTest), in opfs-filesystem-e2e.ts:

• one suite componentizes/transpiles a small wasm component, runs it against a live OpfsFilesystemAdapter, flushes, reloads the same OPFS directory into a fresh adapter, and re-runs the component — confirming writes/symlinks made across the component boundary actually survive in real OPFS storage, not just in one adapter's in-memory tree.
• a second suite exercises cross-tab locking directly against the browser's real navigator.locks (no wasm component needed there) — verifying two independent adapters actually queue/grant through the browser's own Web Locks manager, not a mock.

[vados-cosmonic]

LGTM 🚀

I think some of the new tests could do without the beforeAll (since they are testing results from a single run), but that can definitely get fixed in a follow up!)