feat(ts): add runtime config ramp controls

[lan17]

Summary

• add runtime config resolution for per-layer TTL and ramp controls
• fall back from provider null to cached-function defaultConfig
• disable only the relevant layer for missing/invalid TTL or ramp config, then continue through the remaining cache chain
• add injectable percentage sampling so ramp behavior is deterministic in tests
• document Milestone 3 scope and add Given/When/Then behavioral tests

Validation

• pnpm install --frozen-lockfile
• pnpm ts:gcache:typecheck
• pnpm ts:gcache:test — 34 tests, coverage: 96.53% statements / 95.1% branches / 96.82% functions / 96.88% lines
• pnpm ts:gcache:build

Stack

Stacked on #134 / lev/ts-m2-redis-ttl. This PR intentionally leaves Prometheus metrics, targeted invalidation/watermarks, and framework integrations for later milestone PRs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (1f32c9c) to head (41a08a5).

Additional details and impacted files

@@                 Coverage Diff                  @@
##           lev/ts-m2-redis-ttl     #135   +/-   ##
====================================================
  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.