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

tests/test_swap_writer.cc

@@ -123,7 +123,7 @@ TEST(SwapWriter_Header_Records_And_CRC)
 	static const std::uint8_t magic[8] = {'K', 'T', 'E', '_', 'S', 'W', 'P', '\0'};
 	for (int i = 0; i < 8; ++i)
 		ASSERT_EQ(bytes[(std::size_t) i], magic[i]);
-	ASSERT_EQ(read_le32(bytes.data() + 8), (std::uint32_t) 1);
+	ASSERT_EQ(read_le32(bytes.data() + 8), (std::uint32_t) 2); // v2: chunked checkpoints
 	// flags bit 0: the base file's identity (size, mtime) is recorded, since
 	// the buffer is file-backed; bit 1: its content CRC-32 too.
 	ASSERT_EQ(read_le32(bytes.data() + 12), (std::uint32_t) 3);