perf: cache smart routing decisions

[saurabhhhcodes]

Summary

• add bounded, TTL-based routing decision caching for repeated failure/prompt contexts
• reduce repeated lookup work in fallback/context/cost routing strategies and error classification
• document the new performance options and add a benchmark-style regression test for the <10ms overhead target

Validation

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

Closes #37

[saurabhhhcodes]

Quick scoring-label request: this PR closes GSSoC issue #37, which is labeled gssoc'26, level2, and feature: smart-routing. Could you please mirror the applicable GSSoC/difficulty/type labels onto the PR before merge?