Execute the V1 to V3 migration end-to-end in tests

[bradleymackey]

Problem

The V1 → V3 schema migration — the path that moves plaintext killphrases and search passphrases out of the store and into salted-HMAC digests — was never actually executed by any test. PersistedSchemaMigrationPlanTests asserts only the declaration shape (two custom stages, willMigrate != nil), and every test ModelContainer in the repo omits migrationPlan:. The willMigrate closures, the pending-rehash sidecar files, and the Phase B rehash-on-first-unlock were all unexercised, including the documented plaintext-on-disk window between the phases.

Changes (test-only)

New PersistedSchemaMigrationExecutionTests — builds a real on-disk V1 store with plaintext phrases, reopens it through the real PersistedSchemaMigrationPlan exactly as the production opener does, then runs the real rehash services:

• v1ToV3_migration_writesPendingSidecarsForNonBlankPhrases — willMigrate snapshots exactly the non-blank (itemID, phrase) pairs into both sidecar files.
• v1ToV3_migration_skipsBlankAndNilPhrases — no sidecar entries for nil/empty phrases.
• rehashServices_consumeSidecarsAndDigestsVerifyOriginalPhrases — after KillphraseRehashService/SearchPassphraseRehashService run: sidecars are consumed (securely cleared), killphrase deletion fires with the original phrase (the only public observation point for killphrase digests — deliberate, MANIFESTO C5), and the passphrase-hidden item is unreachable without the matcher but returned with it.
• rehashServices_idempotentWhenSidecarMissing — writer never invoked when there is nothing pending.

Each test gets its own temp directory (created in init, removed in deinit) to avoid cross-test flake. One real-world catch surfaced while writing these: the persisted visibility/searchableLevel strings are the VaultEncodingConstants values ("ALWAYS", "ONLY_PASSPHRASE"), not the Swift enum case names — the tests now seed with the real constants.

Local verification: suite passes, full VaultFeedTests scheme green on iPhone 18 Pro Max / iOS 27.0.

⚠️ Automatic CI is still disabled (#548), so this is local verification only.

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