Swap: fsync without blocking edits; base identity off the lock; races

- The writer's periodic fsync now holds a separate io_mtx_, taken also
  (before mtx_) by the main-thread paths that close journal fds, instead
  of mtx_. Every keystroke takes mtx_ to enqueue its record, so a slow
  fsync (writeback pressure, NFS) froze typing.
- A journal's base identity (stat + content CRC) is computed before
  taking mtx_ and applied under it; the CRC is skipped above 64 MiB
  (size and mtime still identify the file). It re-read the whole file
  under the lock on every open and save.
- A buffer that was never journaled (a virtual buffer saved under a
  real name) is attached by NotifyFilenameChanged.
- ThreadSanitizer: open_ctx (writer thread) no longer writes ctx.path,
  and locked_out is recorded by the caller under mtx_; both raced with
  main-thread reads.

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

tests/test_audit_regressions.cc

@@ -1126,3 +1126,23 @@ TEST(Audit_Save_HardLink_LeftoverCopyDoesNotBlock)
 	ASSERT_TRUE(b.Save(err));
 	ASSERT_EQ(slurp(d.path / "h2.txt"), std::string("new old\n"));
 }
+
+
+// A virtual buffer saved under a real name is journaled from then on.
+TEST(Audit_VirtualBuffer_SavedAs_GetsJournal)
+{
+	TempDir d("virtual_saveas");
+	TestHarness h;
+	Editor &ed = h.EditorRef();
+	ASSERT_TRUE(h.Exec(CommandId::ShowHelp));
+	ASSERT_TRUE(h.Exec(CommandId::ToggleReadOnly));
+	const std::string target = (d.path / "notes.txt").string();
+	ASSERT_TRUE(Execute(ed, "save-as", target));
+	Buffer *b = ed.CurrentBuffer();
+	ASSERT_TRUE(!b->IsVirtual());
+	ASSERT_TRUE(h.Exec(CommandId::InsertText, "x"));
+	ed.Swap()->Flush(b);
+	const std::string swp = kte::SwapManager::ComputeSwapPathForTests(*b);
+	ASSERT_TRUE(std::filesystem::exists(swp));
+	std::filesystem::remove(swp);
+}