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/TestRunner.cc

@@ -1,17 +1,32 @@
 #include "Test.h"
 #include <iostream>
 #include <chrono>
+#include <string>
+#include <vector>
 
 
+// Usage: kte_tests [substring...] runs the tests whose names contain any of
+// the substrings (all tests when none are given).
 int
-main()
+main(int argc, char **argv)
 {
 	using namespace std::chrono;
 	auto &reg = ktet::registry();
+	std::vector<std::string> filters(argv + 1, argv + argc);
+	auto selected = [&filters](const std::string &name) {
+		if (filters.empty())
+			return true;
+		for (const auto &f: filters)
+			if (name.find(f) != std::string::npos)
+				return true;
+		return false;
+	};
 	std::cout << "kte unit tests: " << reg.size() << " test(s)\n";
 	int failed = 0;
 	auto t0    = steady_clock::now();
 	for (const auto &tc: reg) {
+		if (!selected(tc.name))
+			continue;
 		auto ts = steady_clock::now();
 		try {
 			tc.fn();