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

tests/test_audit_regressions.cc

@@ -8,6 +8,7 @@
 
 #include "ErrorRecovery.h"
 #include "PieceTable.h"
+#include "RegexGuard.h"
 #include "UndoTree.h"
 #include "syntax/HighlighterEngine.h"
 #include "syntax/HighlighterRegistry.h"
@@ -880,3 +881,20 @@ TEST(Audit_Commands_FilesystemErrorsDoNotThrow)
 	(void) ed.ProcessPendingOpens();
 	ASSERT_EQ(h.Text(), std::string("precious\n"));
 }
+
+
+// Renderer regex highlighting on a long line with a deeply recursive pattern
+// (overflowed an 8 MiB stack at under 10,000 bytes) runs off the main stack.
+TEST(Audit_ForEachRegexMatch_LongLine)
+{
+	std::string line;
+	while (line.size() < 200000)
+		line += "word, more words; and so on. ";
+	std::size_t n = 0, total = 0;
+	kte::ForEachRegexMatch(line, std::regex("(\\w|\\s|[.,;])+"), [&](std::size_t, std::size_t len) {
+		++n;
+		total += len;
+	});
+	ASSERT_EQ(n, (std::size_t) 1);
+	ASSERT_EQ(total, line.size());
+}