docs(vault): remove drift tracking artifacts, bump vault docs to stable

The vault spec-to-implementation sync is complete. Remove the drift
tracking tools that were only needed during sync:

- Remove the Known Source Drift table from vault/README.md
- Remove 'known drift' / 'current source uses X' prose from Security
  Constraints sections in vault/README.md, encryption.md, and service.md.
  The permanent constraint statements (OsRng for IVs, zeroized drop,
  no unwrap, etc.) are preserved.
- Remove the drift paragraph in encryption.md Key Versioning.
- Remove stale 'to be updated per ADR-025' / 'postcard tests to be
  removed' notes in protocol.md References.
- Bump status: draft -> stable in the frontmatter of all vault docs
  (README, mnemonic-derivation, encryption, service, protocol).
- Update architecture/README.md: vault doc status entries to stable,
  Current State paragraph reflects vault implementation complete (no
  'pending ADR-025/026 refactor' language).

docs/architecture/crates/vault/protocol.md

@@ -1,5 +1,5 @@
 ---
-status: draft
+status: stable
 last_updated: 2026-06-23
 ---
 
@@ -236,9 +236,8 @@ ADR-025 and [open-questions.md](../../open-questions.md).
 
 ## References
 
-- Implementation: `crates/alknet-vault/src/protocol.rs` (to be updated
-  per ADR-025 — remove `VaultProtocol` enum and irpc usage)
+- Implementation: `crates/alknet-vault/src/protocol.rs`
 - Tests: `crates/alknet-vault/src/protocol.rs` (unit tests for redaction
-  and zeroize behavior; postcard tests to be removed)
+  and zeroize behavior)
 - [service.md](service.md) — `VaultServiceHandle` runtime API
 - [mnemonic-derivation.md](mnemonic-derivation.md) — what `KeyType` means