Durable Object RPC Rate Limit

A globally consistent token bucket rate limiter, one Durable Object per API key, consumed over native RPC rather than fetch: the fronting Worker calls take() and peek() directly on a stub from env.LIMITER.getByName(apiKey), so each key shards to its own strongly consistent bucket worldwide. Bucket state lives in DO SQLite and survives eviction, the schema is initialized inside blockConcurrencyWhile and refilled inside transactionSync, and refill is alarm driven: an alarm is armed only while the bucket is below capacity and goes quiet once full, so an idle key costs nothing. The Worker maps the RPC result to RateLimit-Limit, RateLimit-Remaining, RateLimit-Reset, and Retry-After headers. This is the accounting-correct counterpart to the eventually consistent Rate Limiting binding.