Swap ownership, fsync race, RowsView memory, renderer regex stack

- A session never unlinks a journal it does not own: Detach, ResetJournal
  and NotifyFilenameChanged skip removal when this session was locked
  out or another session holds the lock, and PruneSwapDir skips held
  journals. A second session used to delete the first session's live
  journal on save or close.
- The writer thread's periodic fsync now runs under mtx_. It fsynced fds
  that the main thread could close concurrently (or that had been
  reused); an in-flight fsync also kept a just-closed journal's flock
  alive, so the owner's own cleanup saw its journal as held.
- Compaction locks the compacted file before renaming it into place and
  keeps using it, so the journal is never unlocked in between; a failed
  lock on the truncating reopen marks the session locked out.
- Unnamed-buffer journals include the pid, so sessions no longer share
  (and delete) each other's unnamed-N.swp.
- RowsView keeps only the previous generation of cached rows: keeping
  all of them made C-u 2000 M-d on a 1 MiB line use ~2 GB.
- Renderers run regex match highlighting through ForEachRegexMatch,
  which uses the large-stack worker for lines over 512 bytes; some
  patterns overflowed the main stack well under the previous 10,000
  byte limit.

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

QtFrontend.cc

@@ -166,21 +166,13 @@ protected:
 						    (ed_->CurrentPromptKind() == Editor::PromptKind::RegexSearch ||
 						     ed_->CurrentPromptKind() ==
 						     Editor::PromptKind::RegexReplaceFind)) {
-							// Long lines are not highlighted: std::regex recursion
-							// could overflow the stack (RegexGuard.h).
+							// std::regex recursion is kept off the main stack for long
+							// lines (RegexGuard.h); very long lines are not highlighted.
 							if (line.size() <= kte::kRegexRenderLineLimit) try {
 								std::regex rx(ed_->SearchQuery());
-								for (auto it = std::sregex_iterator(
-									     line.begin(), line.end(), rx);
-								     it != std::sregex_iterator(); ++it) {
-									const auto &m  = *it;
-									std::size_t sx = static_cast<std::size_t>(m.
-										position());
-									std::size_t ex =
-										sx + static_cast<std::size_t>(m.
-											length());
-									hl_src_ranges.emplace_back(sx, ex);
-								}
+								kte::ForEachRegexMatch(line, rx, [&](std::size_t pos, std::size_t len) {
+									hl_src_ranges.emplace_back(pos, pos + len);
+								});
 							} catch (const std::regex_error &) {
 								// Invalid regex: ignore, status line already shows errors
 							}