docs(ext/sw): tighten slice-3 comments per code review

Non-functional tightening flagged in the slice-3 code review:

- session.ts: document future multi-vault refactor (β+) so the module-
  scope singleton is explicitly "deliberately simple," not an oversight.
- vault.ts: move findByHostname doc comment above the function; note
  α's intentionally-coarse hostname match (no www-stripping, no
  public-suffix matching) and that tighter matching is a β/γ concern.
- index.ts: expand the passphrase scope-clearing comment to make
  the theatre explicit rather than leaving it looking like real defense.
- index.ts: TODO(slice-4) marker on delete_item's non-atomic two-write
  path — consider manifest-first ordering or retry/rollback at router-
  split time.
- index.ts: cross-reference comment on itemToManifestEntry pointing at
  the Rust-side ManifestEntry::from_item derivation it must mirror.

No behavior change; build still compiles with 2 bundle-size warnings.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

extension/src/service-worker/index.ts

@@ -122,8 +122,12 @@ async function handleMessage(req: Request): Promise<Response> {
 
       const handle = w.unlock(req.passphrase, imageBytes, meta.salt, meta.paramsJson);
       session.setCurrent(handle);
-      // Clear passphrase from scope best-effort.
-      // (JS strings are immutable; the message object goes out of scope after return.)
+      // Best-effort scope clearing. JS strings are immutable so this only
+      // nulls the one reference on `req`; wasm-bindgen already copied the
+      // string into WASM linear memory before `unlock` returned. Left in
+      // as a marker — future work may consider a direct-stream KDF that
+      // never materializes the string, or a `Zeroizing` wrapper on the
+      // WASM-side incoming buffer.
       (req as { passphrase: string }).passphrase = '';
 
       manifest = await vault.fetchAndDecryptManifest(git, handle);
@@ -178,6 +182,10 @@ async function handleMessage(req: Request): Promise<Response> {
       const entry = manifest.items[req.id];
       if (!entry) return { ok: false, error: 'item_not_found' };
       // Soft-delete: fetch the item, set trashed_at, write it back.
+      // TODO(slice-4): not atomic across the two git writes. If the manifest
+      // write fails after the item write, next sync restores the live manifest
+      // and the trashed item re-appears. Consider manifest-first write order
+      // or a retry/rollback pass when router-splitting this handler.
       const item = await vault.fetchAndDecryptItem(gitHost, handle, req.id);
       const now = Math.floor(Date.now() / 1000);
       const updated: Item = { ...item, trashed_at: now, modified: now };
@@ -253,6 +261,12 @@ async function handleMessage(req: Request): Promise<Response> {
   }
 }
 
+/// TS mirror of the Rust core's `ManifestEntry::from_item`
+/// (see crates/relicario-core/src/manifest.rs). These two derivations must
+/// stay in sync — if the Rust side learns to handle e.g. trailing-whitespace
+/// URL parsing differently, this function drifts and the manifest diverges
+/// between CLI-written and extension-written items. A future WASM helper
+/// `item_to_manifest_entry(handle, item_json)` would eliminate the duplication.
 function itemToManifestEntry(item: Item) {
   return {
     id: item.id,