feat: M4 Session Manager + demo

ShadeSessionManager wraps X3DH + Double Ratchet into a simple API:
- initialize(), createPreKeyBundle(), encrypt(), decrypt()
- Automatic PreKeyMessage for first message, RatchetMessage after
- Signed prekey rotation, multi-peer sessions, one-time prekey mgmt
- Interactive demo.ts showing full frontend↔backend E2EE flow

80 tests, 0 failures across M1-M4.

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

packages/shade-core/src/index.ts

@@ -5,3 +5,4 @@ export * from './keys.js';
 export * from './errors.js';
 export * from './x3dh.js';
 export * from './ratchet.js';
+export { ShadeSessionManager } from './session.js';