Swap: chunked checkpoints for large buffers; scaled thresholds; single replay

- A checkpoint larger than one record (16 MiB) is written as CHKPT_BEGIN
  [size u64, CRC-32], CHKPT_DATA chunks and CHKPT_END. Replay assembles it
  and applies it only when complete with the size and CRC matching; an
  unfinished one is a torn tail (valid_bytes stops before it). Buffers
  over 16 MiB could not be checkpointed before: a lost record left their
  journal behind for good, and they were never compacted. Records are
  written as segments straight from the snapshot. Header version 2;
  version 1 journals are still read.
- Checkpoint thresholds scale with the buffer size (edits of half its
  size; the timer only after a sixteenth; compaction at twice its size;
  gap retries spaced by size), and a checkpoint that would compact is
  written straight into the compacted journal instead of being appended
  and then copied.
- The recovery prompt keeps the content it replayed for its preview and
  installs it on 'y', instead of replaying the journal a second time.
- A base file over 64 MiB gets its CRC from the writer thread before it
  creates the journal (it had none, so touching such a file made its
  journal unusable for recovery).
- kte_tests takes optional name substrings to run a subset of tests.

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

Editor.h

@@ -717,6 +717,11 @@ private:
 	PendingOpen pending_recovery_open_{};
 	std::string pending_recovery_swap_path_;
 	std::string pending_recovery_replay_err_;
+	// The recovered content the prompt offers, and how much of the journal
+	// produced it: "y" installs it instead of replaying the journal again.
+	std::string pending_recovery_content_;
+	std::uint64_t pending_recovery_valid_bytes_ = 0;
+	bool pending_recovery_have_content_         = false;
 
 	// GUI-only state (safe no-op in terminal builds)
 	bool file_picker_visible_ = false;