Cache-first seeding and paging

[zealsprince]

The data path that makes the cache visible: today every channel open waits on the network. Wire the spec's flow: on open, render the newest CACHE_SEED_COUNT from cache immediately, then fetch only the delta after the cached tip and reconcile. Scroll-up pages from cache until exhausted, then CHATHISTORY BEFORE the oldest cached msgid, writing pages back. Forward paging with the tailTrimmed flag keeps the live window bounded both directions.

Spec

• Local Cache - prefill, paging, the sliding window

Acceptance criteria

• A cached channel paints from disk without a network wait, then converges with the server delta
• A cold channel falls back to a normal chathistory seed and populates the cache
• Scroll-up serves cache pages until exhausted, then fetches BEFORE and caches the result so the next scroll is local
• Scrolling back down after deep scrollback restores newer messages from cache without refetching
• The live window never exceeds MAX_LIVE_MESSAGES regardless of scroll depth

Blocked by the IndexedDB adapter and writer tickets in this epic, and #33.