feat: add model fallback on provider errors (v0.4.0)

- Add fallback chain execution when primary model fails with provider error
- Detect billing errors, rate limits, model unavailable, and server errors
- Try up to 3 models from tier's fallback chain before returning error
- Add skipBalanceCheck option for testing
- Add fallback.ts test suite (16 tests)
- Add getProxyPort() and checkExistingProxy() helpers

Author: 1bcMax

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@blockrun/clawrouter",
-  "version": "0.3.41",
+  "version": "0.4.0",
   "description": "Smart LLM router — save 78% on inference costs. 30+ models, one wallet, x402 micropayments.",
   "type": "module",
   "main": "dist/index.js",