Fix round-four regressions: save fallback, replace memory, regex skip

- Save's in-place fallbacks (no temp file; no hard-link copy) now apply
  only when the directory refuses a new name (EACCES, EPERM,
  ENAMETOOLONG). On ENOSPC, EDQUOT or EIO the in-place write truncated
  the file and then failed, leaving it corrupt; now the save fails and
  the file is untouched (checked on a full tmpfs).
- replace_rows_text/replace_buffer_text delete and insert only the span
  between the old and new text's common prefix and suffix (kept on
  UTF-8 boundaries). Replace-all recorded the whole buffer twice in the
  undo tree per run.
- Regex search skips long lines only while typing (limit lowered to
  2,000 bytes, as for renderer highlighting; '.*q' on a 20 KB line took
  12 s), reports how many were skipped, and next/previous match search
  every line. Previously matches on long lines could not be found.

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

RegexGuard.h

@@ -13,11 +13,12 @@
 
 namespace kte {
 // Renderers highlight regex matches only on lines up to this many bytes, and
-// incremental regex search skips longer lines: std::regex can take time
-// quadratic in line length even for ordinary patterns, with no way to
-// interrupt it, and these run on every frame or keystroke.
-constexpr std::size_t kRegexRenderLineLimit      = 10000;
-constexpr std::size_t kRegexIncrementalLineLimit = 20000;
+// search-as-you-type skips longer lines (an explicit search does not):
+// std::regex can take time quadratic in line length even for ordinary
+// patterns (".*q" on a 20 KB line: 12 s), with no way to interrupt it, and
+// these run on every frame or keystroke.
+constexpr std::size_t kRegexRenderLineLimit      = 2000;
+constexpr std::size_t kRegexIncrementalLineLimit = 2000;
 
 // Run fn to completion on a thread with a large stack (address space is
 // reserved, pages are committed only as used), so regex work over long lines