Switching feeds paints posts at once instead of a spinner while the relays answer.

The disk cache held four feeds, so with more chips than that (or one never
opened here) a switch opened on nothing. Now:
- up to twelve feeds on disk: the four read last keep a page of 30, the
  rest 5 seed posts (same 256 KB budget);
- once the feed on screen settles, every other feed with nothing on disk
  has its first posts fetched ahead, one small request each, every 15 min;
- a feed with nothing on disk borrows matching posts (its people, its
  topics) that open feeds already hold.

tools/feed-switch-seed-test.js: with 8 s relays, a fetched-ahead feed and a
never-opened topic feed both paint rows at once (fails on the old code).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WMMwWYJkPQxLgfMsY7hWcU

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