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/index.ts

@@ -0,0 +1,7 @@
+export * from './types.js';
+export * from './crypto.js';
+export * from './storage.js';
+export * from './keys.js';
+export * from './errors.js';
+export * from './x3dh.js';
+export * from './ratchet.js';