Implement the Connection Vault protocol layer (phases A-C)

connection-vault.md 5-13 as code, with the implementation-time decisions
recorded in a new section 18 and vault-ux.md holding the agreed UX
walkthroughs as the authorization layer's acceptance criteria.

Capsules gain paired optional fields connection_vault_root and
vault_sudo_key (both or neither, validated); registration mints them,
login/change-password/recovery carry them forward, and
enableConnectionVault is the phrase-gated migration for existing
accounts -- recovery capsule written first, fail closed, idempotent by
reusing roots found in either capsule so a partial failure can never
mint twice and strand records.

src/vault/ holds the finalized key hierarchy (HKDF/ScalarExpand
constants pinned by test vectors that must only ever be reverted, never
updated), per-connection encrypted records signed by a derived vault
identity with the capsule JCS+padding+AES-GCM pipeline and shared
buckets so record types are indistinguishable by size, the NWC profile
with lossless URI parse/export, relay sync reusing the publish-and-
verify quorum bar, and per-record high-water marks so a stale replica
is withheld and reported rather than silently resurrecting a revoked
credential.

The two-tier model makes sudo mode cryptographically real: personal-
tier record keys require vault_sudo_key as HKDF input, VaultSession
holds it only inside an enableSudo/endSudo window (wiping on the way in
and out), and outside a window personal records are invisible -- the
same trial-decrypt returns null, not an error. The app-facing API is
structurally blind to the tier by construction.

118 tests passing, including four end-to-end scenarios against mock
relays: clean-device restore, password-change carry, legacy-account
migration (wrong phrase refused), and rollback detection.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01CMr9Yt8SYmZcfgrg3CV2G1

docs/README.md

@@ -33,14 +33,19 @@ identity and recovery model.
 
 ## Connection Vault
 
-The Connection Vault is an experimental design for portable application
-credentials. It deliberately stores mutable credentials outside the core
-identity capsules as independently encrypted records.
+The Connection Vault stores portable application credentials outside the core
+identity capsules as independently encrypted records. Its protocol layer
+(records, key hierarchy, relay sync, migration, tiers) is **implemented in
+`@bitlogin/core`'s `src/vault/`**; the widget authorization layer (consent
+UI, grants, Connected apps) is still to come.
 
 - [`connection-vault.md`](./connection-vault.md) — overall vault architecture,
-  key hierarchy, relay storage, authorization, recovery, and deletion
+  key hierarchy, relay storage, authorization, recovery, and deletion; §18
+  records the implementation-time decisions (tier model, sudo key)
+- [`vault-ux.md`](./vault-ux.md) — the agreed user-experience walkthroughs,
+  serving as the authorization layer's acceptance criteria
 - [`nwc-connections.md`](./nwc-connections.md) — Nostr Wallet Connect credential
-  profile
+  profile (parser/exporter implemented in `src/vault/nwc.ts`)
 - [`s3c-format.md`](./s3c-format.md) — portable `s3c1:` format for S3-compatible
   storage connections