README.md and site/data.json were written with os.Create/os.WriteFile,
so a write that failed midway left the repo front page truncated, while
history.jsonl already used a temp-file-plus-rename. Extract that pattern
into atomicWriteFile and route all three writers through it.
Also sort snapshots by date when reading history.jsonl: delta windows
pick the newest snapshot inside the window by scan order, which silently
produces wrong deltas if a hand edit or a merge of two concurrent runs
interleaves lines.
Cover the README renderer, which had no test beyond sanitizeCell, and
name the generated paths as constants instead of repeating literals.
Coverage 65.1% -> 75.7%.
