In memory cache layer

[gloskull]

Motivation
Provide a two-tier caching solution to keep read-heavy contract paths (meter status and usage rollups) fast and to meet the <100ms P99 target for warm reads.
Allow optional shared coherence via Redis for multi-process deployments while keeping a safe in-process hot path for local development and tests.
Expose TTLs and runtime metrics so the service can be monitored and operated safely during blue/green and canary rollouts.
Description
Adds a new cache implementation meter-simulator/src/cache-layer.js implementing an in-process memory hot cache plus an optional minimal Redis RESP client and runtime metrics (cache_hits, cache_misses, cache_sets, cache_deletes, cache_errors).
Wires the cache into ContractInterface so getMeter and getUsageData use cache.remember(...) and so submitUsageData/submitZkUsageData call _invalidateMeterCache(...) to evict meter and usage entries after writes.
Adds cache configuration to meter-simulator/src/config.js, a cache-info CLI command and cache metrics output in the status command in meter-simulator/src/index.js, and documents architecture/operational guidance in docs/REDIS_CACHE_ARCHITECTURE.md and meter-simulator/README.md.
Adds focused Jest tests meter-simulator/tests/cache-layer.test.js and meter-simulator/tests/contract-cache.test.js that cover memory hits, TTL expiry, deletion, caching behavior in ContractInterface, and cache invalidation after usage submission.
Testing
Ran focused unit tests with npm test --prefix meter-simulator -- tests/cache-layer.test.js tests/contract-cache.test.js --runInBand, which passed (both new test suites green).
Performed static checks with node --check meter-simulator/src/cache-layer.js, node --check meter-simulator/src/contract-interface.js, and node --check meter-simulator/src/index.js, which reported no syntax errors.
Ran the full simulator test run npm test --prefix meter-simulator -- --runInBand; the new cache tests pass but the pre-existing tests/meter-device.test.js failed due to unrelated expectations in that suite (invalid mock key material and time/message-length assertions), so full test suite remains failing for reasons unrelated to the cache changes.
Closes #70