t27b corpus --reference-cache: interleaved rows and an mtime-keyed cache #6332

Seen by the t27b char-literal lane (#6331), Refs #6063.

t27b corpus --reference-cache <file>:

1. Worker threads append cache rows with writeln!, which can split one row into several writes. One damaged line in the cache had two rows interleaved. The reader skips damaged lines, so the only cost is a lost cache entry, but a cache should not depend on that. Fix: format each row into one buffer and write it with a single write_all, or send rows to one writer thread.
2. The cache key includes the t27c binary's mtime, so a copied t27c (the lanes keep fixed copies so they can rebuild while measuring) misses the whole cache. Key on the binary's content hash (or t27c --version plus its size and hash) instead.

Also seen, not confirmed: a probe reported a slice out-of-bounds trap at line 5 when the slice was on line 7. That needs a minimal repro before it becomes its own issue.