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/session.ts

@@ -3,6 +3,10 @@
 /// α assumes one vault per extension install. The master key lives only
 /// inside WASM linear memory (wrapped in Zeroizing<[u8;32]>); this module
 /// just holds the opaque handle that names it.
+///
+/// Future multi-vault (β+) would replace `current` with
+/// `Map<vaultId, SessionHandle>` and thread `vaultId` through every
+/// handler. Deliberate α simplicity — not an oversight.
 
 import type { SessionHandle } from '../../wasm/relicario_wasm';