Swap: retry a journal's recovery checkpoint while idle

After a lost record the journal drops position-based records until a
full checkpoint. That checkpoint was only requested from later edits,
rate-limited to once a second; when the writer handled the failing
record before its edit asked for a checkpoint, the checkpoint failed as
well and consumed the slot, so edits made within the next second were
not recovered and, if the user stopped typing, the journal stayed behind
indefinitely. SwapManager::RetryGapCheckpoints(), called once per frame
from Editor::ProcessPendingOpens, requests it once the interval has
passed. This also made SwapReplay_LostRecord_ResyncsWithCheckpoint flaky
under load; the test now exercises the retry.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

Swap.cc

@@ -1172,6 +1172,27 @@ SwapManager::maybe_request_checkpoint(Buffer &buf, const std::size_t approx_edit
 }
 
 
+void
+SwapManager::RetryGapCheckpoints()
+{
+	std::vector<Buffer *> due;
+	{
+		std::lock_guard<std::mutex> lg(mtx_);
+		const std::uint64_t now = now_ns();
+		for (auto &[b, ctx]: journals_) {
+			if (!b || !ctx.gap || ctx.suspended || ctx.locked_out || ctx.gap_unfixable_reported)
+				continue;
+			if (now - ctx.gap_chkpt_request_ns < kGapCheckpointIntervalNs)
+				continue;
+			ctx.gap_chkpt_request_ns = now;
+			due.push_back(b);
+		}
+	}
+	for (Buffer *b: due)
+		RecordCheckpoint(*b, false);
+}
+
+
 void
 SwapManager::RecordCheckpoint(Buffer &buf, const bool urgent_flush)
 {