foldToSnapshot overwrites work.json after an unparseable snapshot or unreadable event log, with nothing recorded #2513

What happened

foldToSnapshot() publishes work.json from readSnapshot() + replayOntoSnapshot(). Both helpers turn "could not read" into "empty/unchanged" without recording it, and the fold then writes the result with a fresh _folded_at:

• work.json exists but does not parse -> readSnapshot returns { sessions: {} }. After a compaction (log truncated to the suffix, older log in the archive directory), the fold writes a snapshot that only holds sessions from the surviving suffix. Everything folded before the compaction is gone from work.json, and nothing says so.
• The event log cannot be stat'ed (permissions, EIO) -> replayOntoSnapshot returns the snapshot's sessions with offset: 0; the fold writes _events_offset: 0 and _folded_at: now. If the log cannot be read after a successful stat, it returns quietly with the old offset. Either way the snapshot is stamped "folded just now" over an input that was never read.

The header says a skipped fold is safe because "events stay in the log" and the next writer picks them up. That holds for lock contention (acquireLock fails -> return false), not for these two paths, which proceed and write.

Root cause

hooks/lib/work-events.ts:

• :448-455 readSnapshot catch returns an empty registry for any error, not only ENOENT.
• :466-471 and :484-485 replayOntoSnapshot catches return without any report.
• :564-593 foldToSnapshot has no way to know a read failed, so it publishes.

Suggestion

Report the failure and skip the publish. Only ENOENT is a legitimate quiet case.

// replayOntoSnapshot returns a `problem` when stat/read fail with anything but ENOENT
return { registry, offset, problem: `event log unreadable: ${e.code ?? e.message}` };
...
// foldToSnapshot
const { registry, offset, problem } = replayOntoSnapshot(snapshot, logPath);
if (problem) { statvsAnomaly('work-events.fold-skipped', problem, logPath); return false; }

and in readSnapshot, treat a parse error as a problem (anomaly + fold skipped) while ENOENT returns the empty registry. We carry a local patch with this shape for the log; the snapshot side is the same idea. Read, not run.