perf(router): cache repeated routing decisions

[saurabhhhcodes]

Summary

• add a bounded per-router decision cache for repeated routing failure contexts
• expose cache stats and a clear method for instrumentation and tests
• add a routing performance regression test that verifies repeated fallback decisions stay below the 10ms overhead target

Validation

• npm run build
• node tests/routing-performance.test.js
• node tests/cross-provider.test.js
• git diff --check

GSSoC 2026

Fixes #37

[saurabhhhcodes]

Validation complete from my side:

• npm run build
• node tests/routing-performance.test.js -> repeated fallback routing average stayed around 0.0005ms
• node tests/cross-provider.test.js -> 19/19 passed
• git diff --check

If accepted, could you please add the scoring labels from the linked issue: gssoc'26, level2, feature: smart-routing, and a performance/enhancement label?