Persist only changed emulator state and roll up Autumn usage

[RhysSullivan]

Staging billing admission was slow on the shared emulator (UsefulSoftwareCo/executor-next#676).

• Durable Object persistence. After every non-GET request, persist() rewrote the whole store snapshot and up to 1,000 ledger entries, then listed every prefix to delete leftovers. It now keeps the JSON last written for each key, writes only keys that changed, and deletes only keys that went away. A read-only POST such as customers.get_or_create writes its ledger entry and the ledger index only. A rejected write writes nothing. The recorded values are read from storage once per live instance; direct writes (seed, legacy migration) and failed writes reset the record.
• Autumn usage rollup. Every check with send_event added an event, and balances scanned all events. Once a customer has more than 64 events for a feature, adjacent events are merged into one. Events that a subscription's usage watermark or a current reset window separates are never merged, so every balance stays the same. Balance reads now use the customer index instead of scanning every event. events.list returns the rolled-up events (documented in the Autumn skill).

Tests are in #34.

Not deployed. The shared emulator needs a separate approval to merge and redeploy.