feat: M5 Prekey Server + M7 Wire Format

M5: Shade Prekey Server (Hono)
- REST API: register, fetch bundle, replenish, count, delete
- MemoryPrekeyStore for testing/embedded use
- Standalone Docker deployment (Dockerfile + standalone.ts)
- One-time prekey consumption on bundle fetch

M7: Compact binary wire format
- Version-tagged envelopes (PreKeyMessage, RatchetMessage)
- Length-prefixed byte arrays, big-endian integers
- Significantly smaller than JSON (no base64 bloat)
- Roundtrip encode/decode for all message types

100 tests, 0 failures across M1-M5+M7.

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

packages/shade-server/Dockerfile

@@ -0,0 +1,11 @@
+FROM oven/bun:1-alpine
+
+WORKDIR /app
+COPY package.json bun.lock ./
+RUN bun install --production --frozen-lockfile
+
+COPY src/ src/
+
+EXPOSE 3900
+
+CMD ["bun", "run", "src/standalone.ts"]