feat(hardening): M-Hard 1-5 — crypto, auth, rate limit, fingerprints, rotation

M-Hard 1: Cryptographic Hardening
- constantTimeEqual, zeroize, randomUint32 on CryptoProvider
- Fix Math.random() → crypto.randomUint32() for registrationId
- Zero message keys and chain keys after use in ratchet.ts
- Constant-time trust comparison in MemoryStorage + SQLiteStorage
- Timing variance test catches early-exit regressions

M-Hard 2: Self-Authenticated Prekey Server
- Ed25519 signature verification on all write routes
- signPayload/verifyPayload with canonical JSON, ±5 min replay window
- Address validation (NFKC, alphanumeric + :_-.)
- Global ShadeError → HTTP status mapping
- ShadeFetchTransport signs requests when signingPrivateKey provided
- Anonymous bundle fetches still allowed (read-only)

M-Hard 3: Rate Limiting + DoS Protection
- Token bucket rate limiter with pluggable store
- Per-route limits: register 5/h/IP, fetch 60/min/IP, replenish 10/min/id
- 64KB body size limit on POST
- Retry-After header on 429 responses

M-Hard 4: Auto-replenish + Fingerprints + Session Reset
- Safety numbers (12 groups × 5 digits, Signal-style)
- ensurePreKeyStock, resetSession, acceptIdentityChange
- verifyRemoteIdentity for out-of-band comparison

M-Hard 5: Identity Rotation with Grace Period
- rotateIdentity archives old identity, generates fresh signed prekey
- RetiredIdentity storage with addRetired/getRetired/pruneRetired
- 7-day default grace period for decrypting old sessions
- pruneExpiredIdentities for cleanup

M-Hard 8: Error Hierarchy
- New error types: Network, Storage, Validation, Timeout, RateLimit,
  Configuration, Unauthorized, Replay, IdentityRotation
- All errors have stable SHADE_* codes
- errorToHttpStatus for consistent HTTP mapping
- toJSON() for network serialization

188 tests passing, zero failures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shade-core/src/storage.ts

@@ -1,5 +1,11 @@
 import type { IdentityKeyPair, SignedPreKey, OneTimePreKey, SessionState } from './types.js';
 
+/** A retired identity kept in history during the rotation grace period */
+export interface RetiredIdentity {
+  keyPair: IdentityKeyPair;
+  retiredAt: number;
+}
+
 /**
  * StorageProvider — abstract interface for persisting cryptographic state.
  *
@@ -65,4 +71,15 @@ export interface StorageProvider {
 
   /** Save a trusted remote identity key */
   saveTrustedIdentity(address: string, identityKey: Uint8Array): Promise<void>;
+
+  // ─── Identity History (rotation with grace period) ──────
+
+  /** Add an identity to the retired history */
+  addRetiredIdentity(identity: RetiredIdentity): Promise<void>;
+
+  /** Get all retired identities (for grace-period decryption) */
+  getRetiredIdentities(): Promise<RetiredIdentity[]>;
+
+  /** Remove retired identities older than the given timestamp */
+  pruneRetiredIdentities(olderThan: number): Promise<void>;
 }