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/vault.ts

@@ -124,6 +124,13 @@ export function searchItems(
     });
 }
 
+/// Match manifest entries against a page hostname.
+///
+/// icon_hint is derived by the Rust core (crates/relicario-core/src/manifest.rs)
+/// from LoginCore.url's hostname, so equality on icon_hint is the cheapest match.
+/// α is intentionally coarse: no www.-stripping, no public-suffix matching
+/// (`www.github.com` saved items will not match `github.com`, and vice versa).
+/// Tighter matching is a 1C-β/γ concern.
 export function findByHostname(
   manifest: Manifest,
   hostname: string,
@@ -132,6 +139,4 @@ export function findByHostname(
   return (Object.entries(manifest.items) as Array<[ItemId, ManifestEntry]>)
     .filter(([, e]) => e.trashed_at === undefined)
     .filter(([, e]) => (e.icon_hint ?? '').toLowerCase() === h);
-  // icon_hint is derived by Rust core from LoginCore.url's hostname,
-  // so hostname equality on icon_hint is the cheapest match.
 }