sidebar: after a herdr restart, a seen record equal to the new counter quiets an unopened finish #591

From the pre-release Codex (gpt-6-astra) review of v0.4.0..6993563 (finding 6). Confirmed by reading the code; Codex reproduced it with the real functions. Affects only the opt-in Quiet opened finishes setting (#529).

After herdr restarts and keeps pane ids, liveSeqs (src/lib/sidebarOrder.ts) notices the reset when any pane's counter goes back and drops its stand-ins, and pruneSeen drops seen records ABOVE a pane's live counter. A record that happens to EQUAL the new session's counter survives: with seen { a: 2, b: 100 } and a new snapshot { a: 2, b: 1 } where A has a new DONE, B proves the reset but A's old record matches, so A's unopened finish shows as already looked at.

Expected: once a reset is detected for a PC, its whole seen record (in memory and in storage) is invalid. Suggested fix: expose the detected reset from liveSeqs to sidebarActivity and clear that PC's seen record there; or keep a herdr session generation beside the stored record. Add the case above to src/lib/sidebarOrder.test.ts.