1f7edac7

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Mutation redundancy: fix kill-row loss in C-locale sandbox

The mutation child writes its kill row only after printOutputSpecForest.
In a build sandbox with a C/POSIX locale, that printer crashed on the
Unicode result markers (e.g. the ✓ check, '\10003') with
'commitAndReleaseBuffer: invalid argument', so the child exited non-zero
(looking killed) before writing the row — every kill row was lost and the
redundancy analysis came out empty.

- sydtest: force UTF-8 on stdout/stderr in the mutation-child dispatch so
  a result marker can't crash the child.
- driver: ensureDir the out-dir up front, and read the kill row with
  forgivingAbsence so a genuinely-absent row (e.g. on timeout) is skipped
  rather than masked as a killed mutation.