feat(core-shared): add NoopRateLimit and InMemoryRateLimit implementations

NoopRateLimit always allows with Infinity remaining — zero-overhead default
for apps without a wired rate-limit impl. InMemoryRateLimit uses a Map-backed
fixed-window with check-at-read expiry and an injected clock for testability.
Both exported from the core-shared barrel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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