feat(ts): add local-only gcache MVP

[lan17]

Summary

• add a packages/gcache-ts TypeScript package as the Milestone 1 local-only MVP
• implement explicit key builders, AsyncLocalStorage enabled/disabled scopes, local TTL caching, delete/flushAll, and fail-open behavior
• add high-coverage behavioral tests using Given/When/Then comments and a TypeScript CI workflow

Validation

• pnpm install --frozen-lockfile
• pnpm ts:gcache:typecheck
• pnpm ts:gcache:test — 16 tests, coverage: 99.1% statements / 96.36% branches / 100% functions / 99.07% lines
• pnpm ts:gcache:build

Milestone scope

This is Milestone 1 only: a usable local-only async GCache package. Redis, runtime ramp config, Prometheus metrics, invalidation, and framework middleware are intentionally left for stacked follow-up PRs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (683034b) to head (9fc8b7e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #133   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files          13       13           
  Lines         673      673           
=======================================
  Hits          656      656           
  Misses         17       17           

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[lan17]

Closing this stacked milestone PR in favor of the consolidated peak PR #137.