A sync write during a save shows a false 'could not save' Notice #481

When a sync client writes data.json in the short window between Obsidian's writeFile and its utimes during a PodNotes save, PodNotes shows "PodNotes could not save your changes." even though nothing was lost.

Seen in: the SYNC1 (#480) and V-480 stress loops, under heavy CPU load: 17 in 300 runs (SYNC1), 4 in 100 on the fix and 5 in 100 on master (V-480). Every affected run recovered, and since #480 the synced library is adopted at once. The rate under stress doesn't estimate how often real users see it, but it's reachable in a real sync collision.

Cause: PERS3's readback check reads data.json after the save. If another writer replaced it in between, the readback doesn't match and the writer reports a failed save.

Expected: when the readback differs because a newer external version arrived (one the merge would adopt), treat it as an external change, not a failed save, and show no failure Notice. Keep the Notice for real write failures.

Also from V-480, low priority:

• an already open PodNotes settings tab isn't refreshed when Obsidian suppresses its change callback (fix(sync): show a synced library that Obsidian does not report as changed #480 adopts the data, but the tab shows the old values until it's reopened);
• the decode short-circuit added in fix(sync): show a synced library that Obsidian does not report as changed #480 has no automated performance guard.