feat: Shade E2EE library — M1-M3 complete

Signal Protocol implementation with full X3DH + Double Ratchet:

- M1: Core types, CryptoProvider interface, KDF chain functions,
  SubtleCrypto+noble/curves provider, MemoryStorage
- M2: X3DH key agreement (identity keys, signed prekeys, one-time
  prekeys, bundle processing for both initiator and responder)
- M3: Double Ratchet (symmetric-key ratchet, DH ratchet, skipped
  message key cache, out-of-order delivery, AAD-bound headers)

68 tests, 0 failures — including full integration test of
X3DH handshake → Double Ratchet conversation.

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

packages/shade-core/src/storage.ts

@@ -0,0 +1,68 @@
+import type { IdentityKeyPair, SignedPreKey, OneTimePreKey, SessionState } from './types.js';
+
+/**
+ * StorageProvider — abstract interface for persisting cryptographic state.
+ *
+ * Implementations per platform:
+ *  - In-memory (testing)
+ *  - IndexedDB (browser)
+ *  - SQLite/PostgreSQL (server)
+ *  - EncryptedSharedPreferences (Android)
+ */
+export interface StorageProvider {
+  // ─── Identity ──────────────────────────────────────────────
+
+  /** Get our local identity keypair, or null if not yet generated */
+  getIdentityKeyPair(): Promise<IdentityKeyPair | null>;
+
+  /** Persist our local identity keypair */
+  saveIdentityKeyPair(keyPair: IdentityKeyPair): Promise<void>;
+
+  /** Get our local registration ID (unique per installation) */
+  getLocalRegistrationId(): Promise<number>;
+
+  /** Save our local registration ID */
+  saveLocalRegistrationId(id: number): Promise<void>;
+
+  // ─── Signed Pre-Keys ──────────────────────────────────────
+
+  /** Get a signed prekey by ID */
+  getSignedPreKey(keyId: number): Promise<SignedPreKey | null>;
+
+  /** Persist a signed prekey */
+  saveSignedPreKey(key: SignedPreKey): Promise<void>;
+
+  /** Remove a signed prekey (after rotation grace period) */
+  removeSignedPreKey(keyId: number): Promise<void>;
+
+  // ─── One-Time Pre-Keys ────────────────────────────────────
+
+  /** Get a one-time prekey by ID */
+  getOneTimePreKey(keyId: number): Promise<OneTimePreKey | null>;
+
+  /** Persist a one-time prekey */
+  saveOneTimePreKey(key: OneTimePreKey): Promise<void>;
+
+  /** Remove a consumed one-time prekey */
+  removeOneTimePreKey(keyId: number): Promise<void>;
+
+  /** Count remaining one-time prekeys */
+  getOneTimePreKeyCount(): Promise<number>;
+
+  // ─── Sessions ─────────────────────────────────────────────
+
+  /** Get session state for a peer address (e.g. "device:abc123") */
+  getSession(address: string): Promise<SessionState | null>;
+
+  /** Persist session state for a peer */
+  saveSession(address: string, state: SessionState): Promise<void>;
+
+  /** Remove session for a peer */
+  removeSession(address: string): Promise<void>;
+
+  /** Check if we trust a remote identity key (for TOFU or pinned keys) */
+  isTrustedIdentity(address: string, identityKey: Uint8Array): Promise<boolean>;
+
+  /** Save a trusted remote identity key */
+  saveTrustedIdentity(address: string, identityKey: Uint8Array): Promise<void>;
+}