test: a committed mutant list, run nightly

[cubehouse]

The same job the Python SDK now carries (ThemeParks_Python#30), and for the same reason.

An author-written mutant list contains the mutations the author's tests already catch. One scored 18/18 on this package while an independent sweep found ten survivors. That is grading my own homework, and the score was the reassuring kind.

So the list is committed and reviewable — test/mutation/mutants.json, twenty entries, each carrying why it matters. Most name a defect that reached a real file:

• the write error finish() discarded, so ENOSPC printed done: N rows and exited 0 with a truncated file
• the stream with no 'error' listener, which killed a six-park run on the first park
• the checkpoint taken from the newest row rather than the page boundary
• the bare \r that split one CSV row into two
• the CJK query that listed all 127 parks because normalize('東京') is ''

Nightly and non-gating: a survivor is information, not a reason to block a merge. A mutant whose find no longer matches counts as a failure too — a stale mutant has been silently testing nothing, which is the same failure mode as a test that cannot fail.

One mutant targets the generated column list, so the shared csv_contract.json fixture is exercised as a mutation as well as a test: renaming a column there must turn the suite red, or the two SDKs can drift apart again.

20/20 killed.

🤖 Generated with Claude Code