chore: CONTRIBUTING.md and issue/PR templates

[krkarma777]

• CONTRIBUTING.md: dev setup (Node 24 for the TS test suite), `npm run typecheck && npm test` gate, TDD expectation for core changes (the fuzz/optimality/oracle suites are the safety net), bench methodology for perf claims.
• .github/ISSUE_TEMPLATE: bug report (minimal repro: inputs + mode + expected/actual entries) and feature request.
• PR template: checklist mirroring CI (typecheck, tests, bench when perf-related, CHANGELOG entry).