perf: 580x faster embedding init - separate model from WASM

Cloud Run cold starts taking 139 seconds due to 90MB WASM file with
embedded 87MB model weights. WASM compilation scales with file size.

Solution: Split into 2.4MB WASM (code only) + external model files.
- WASM compile: 139,000ms → 6-8ms
- Model load: N/A → 30-115ms
- Total init: 139,000ms → 136-240ms

New modelLoader.ts handles all environments:
- Node.js: fs.readFile()
- Bun: Bun.file()
- Bun --compile: auto-embedded assets
- Browser: fetch()

Zero config - same API, npm package includes model files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -145,6 +145,7 @@
     "dist/**/*.js",
     "dist/**/*.d.ts",
     "dist/**/*.wasm",
+    "assets/models/all-MiniLM-L6-v2/**",
     "bin/",
     "brainy.png",
     "LICENSE",