Debounce channel history persistence

[daiverd]

Fixes #105

useChannelHistory re-serialized every buffer and synchronously wrote about 85 KB (observed live) to localStorage on every channel message.

Changes

• Coalesce saves with a 500 ms trailing debounce, mirroring the output history behavior.
• Defer serialization into the flush so a burst of messages costs one serialization and one write.
• Flush a pending write synchronously on pagehide, beforeunload, and React unmount.
• Cancel the pending timer before flushing so consecutive browser lifecycle events do not duplicate writes.

TDD and verification

• RED: the focused hook suite failed both new pagehide and beforeunload cases because no history was persisted inside the debounce window.
• GREEN: focused hook suite 18/18.
• Full suite: 107 files and 1,065 tests passed.
• TypeScript typecheck passed; staged lint passed with existing warnings only; staged diff check passed.