feat(ts): add redis ttl cache layer

[lan17]

Summary

• add an optional Redis-backed cache layer behind a small command-client interface
• preserve local-only behavior when Redis is not configured, and add local → Redis → fallback read-through when it is
• add versioned timestamped Redis envelopes, default/custom serializer support, cross-layer delete/flushAll, and fail-open Redis error handling
• document Milestone 2 behavior and add Given/When/Then behavioral Redis tests

Validation

• pnpm install --frozen-lockfile
• pnpm ts:gcache:typecheck
• pnpm ts:gcache:test — 26 tests, coverage: 98.13% statements / 95.27% branches / 100% functions / 98.08% lines
• pnpm ts:gcache:build

Stack

Stacked on #133 / lev/ts-m1-local-mvp. This PR intentionally leaves runtime ramp controls, metrics, and targeted invalidation/watermarks for later milestone PRs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (9fc8b7e) to head (1f32c9c).

Additional details and impacted files

@@                 Coverage Diff                  @@
##           lev/ts-m1-local-mvp     #134   +/-   ##
====================================================
  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.