Add TypeScript cache adapter with cross-language conformance tests

Port of Python's origin-side cache to TypeScript:
- cache/keys.ts: deriveCacheKey with stableStringify for JSON-canonical HMAC
- cache/backend.ts: MemoryCache (same API as Python)
- cache/index.ts: cacheGet, cachePut, cachePurge with AND semantics

Integrated into dispatch.ts:
- handleContextFetch: cache lookup before execution, store after
- handleMutationCall: purge on invalidation

Cross-language pin test proves Python and TypeScript produce identical
HMAC-SHA256 output for the same inputs:
  Public:      605a1ca5ad5994e9b765c8d1b330474c2a0d51a7b8fbbdc402f992da7ba902f6
  User-scoped: 30fc08eb46ee4ff2cf7d317e97dca90fd616511e0587304416f71dc863338dc2

34 TypeScript tests (9 new), 165 Python tests (1 new pin test).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/mizan-ts/src/index.ts

@@ -11,3 +11,7 @@ export type { MizanResponse } from './dispatch'
 export { resolveInvalidation, formatInvalidateHeader } from './invalidation'
 
 export { generateManifest } from './manifest'
+
+export { MemoryCache, getCache, setCache, resetCache, cacheGet, cachePut, cachePurge, deriveCacheKey } from './cache'
+export type { CacheBackend } from './cache'
+export { setCacheSecret } from './dispatch'