mise.toml pins zig = "latest" — a floating pin that disagrees with zig.yml #813

What

mise.toml pins zig = "latest".

A floating pin means the toolchain can change under CI without any commit to
this repo, so a green run is not evidence about the next run, and a red one
cannot be attributed to a change anyone made here.

.github/workflows/zig.yml (added by PR #811) pins 0.16.0 explicitly, with
a SHA-256 of the tarball and a test "$(zig version)" = "${ZIG_VERSION}"
assertion. So the repo now states two different Zig versions in two places, and
only one of them is reproducible.

Acceptance criteria

• mise.toml pins an exact Zig version.
• That version equals ZIG_VERSION in .github/workflows/zig.yml.
• A check — a just recipe, a CI step, or a comment naming the other file
  in both places — makes the two impossible to drift apart silently. A
  comment alone is the weakest acceptable form; prefer an assertion.
• Confirm by measurement that the pinned version is the one that produces
  the green result, rather than assuming.