fix(sidebar): write a read finish back when another tab removed it unheard - #1797

…nish-index code (#1539) (#2183)

## Change

Closes #1539, the follow-ups from the merge pass's review of #1536. I
re-checked each item against main 97f957b:

1. **Code kept for no current use** (`src/lib/seenFinishes.ts`,
`src/lib/sidebarActivity.tsx`), removed:
- `hydrate()`'s `if (!refreshing) ... stored = false` loop. Every path
that marks the index unhydrated without a refresh (first load, a
different storage, a storage clear event, the last subscriber leaving)
empties `records` first, so the loop never had anything to reset.
- `cleanSeen()` with no arguments, which forced a fresh storage
snapshot. Only a test called it, and `hydrate()` already removes
old-format and over-limit keys. `retired` is now required, and that test
reaches the same cleanup through `loadSeen()`.
- The `epochs` map's LRU capped at `SEEN_LIMIT`. The map is keyed by
machine ID (one entry per PC in the roster), so it is now a plain `set`.
2. **Past 2,000 records, an old read finish turns unread while you
watch.** This is kept on purpose (agreed with Dori), and a comment on
`trimRecords()` now says so. It takes 2,000 other panes read since that
one. Keeping evicted records for the page's life would bring back the
per-update merge that #1536 removed. After a reload the finish shows
unread anyway, as it did before.
3. **Stale cache with no subscriber mounted:** already fixed by #1797.
4. **A failed share is not retried if the answer fingerprint changes
before the retry:** already fixed by #1747, which made retries
timer-driven. A same-ID entry keeps its pending retry, and a newly read
answer replaces the entry with one that waits for the same timer. On
current main a same-ID fingerprint can only go from "none" to an answer:
Codex re-dating is accepted without changing the remembered answer. A
new test pins that this retry happens once and carries the answer.
5. **A PC with no finishes is not cached:** `recordFor()` now caches the
empty record as well, so each roster update no longer walks up to 2,000
records for such a PC. The empty record is invalidated wherever a
non-empty one is: `cacheChange` on a new ID, `trimRecords`, `hydrate`,
storage events, storage loss and the last unsubscribe. `cleanSeen()`
also drops retired scopes from that cache. The audit doc no longer cites
a `/tmp` path for its disposable harness.

No visual change, no new setting, and no change to the HTTP/WebSocket
contract or the stored record format. **Design precedent:** none in the
UI; this is internal bookkeeping.

## Validation

- New `seenFinishes.test.ts` case "walks the index once for a PC with no
finishes, and sees the first finish written for it". It **failed on
main** (`Map.prototype.values` called once on a repeated empty lookup)
and passes now. It also checks that the first `saveSeen` and
`receiveSeen` into that scope show up.
- New hook case "retries a failed share once, with the answer
fingerprint that arrived before the retry". It passes on main and after.
It fails when the retry is dropped while a backoff timer is pending
(mutation checked, then restored).
- New hook case "does not cache a retired epoch's scope again while it
retires" (the Codex review's one LOW). Without the second commit,
`cleanSeen()`'s notification ran the subscriber's state update at once,
and that update read the retired epoch and cached its scope again: one
entry per epoch change until the last subscriber left. No dot was wrong.
The fix records the new epoch before `cleanSeen()` and makes the
subscriber skip an epoch that is no longer the PC's current one. This
test **failed before that fix** and passes now.
- `bun test src/lib/seenFinishes.test.ts
src/lib/sidebarActivity.hook.test.ts src/lib/panePresentation.test.ts`:
78 pass, 0 fail.
- `bun run check fast` on the head: passes (workflow syntax, generated
types and themes, typecheck, build, 7929 unit tests with 0 failures).
- `bun run check run --build bun
scripts/sidebar-activity-demo-regression.ts` on the head: 44 scenarios
pass, 0 fail.
- Reviews: omo-native-code-reviewer PASS on the first commit (three LOW
notes, none needing a change). Codex Sol 6.1 Fast xhigh found one LOW
(the retired-scope entry above), and its re-review of the fix passed
with no findings.

The items above were re-checked against main 97f957b; the three commits
main gained since touch none of these files.