Catch missing msgfmt and harden the Metal Toolchain probe in doctor

[sbertix]

Closes #729

Summary

make doctor reported all checks green while make build-app then failed on
prerequisites the doctor never verified. Two gaps, both in scripts/doctor.sh:

• msgfmt (GNU gettext) was unchecked. Ghostty shells out to msgfmt to
  compile its .po translation catalogs (ThirdParty/ghostty/src/build/GhosttyI18n.zig:33),
  and macOS does not ship it, so a build without it fails with
  failed to spawn ... msgfmt: FileNotFound (once per locale). Doctor now
  checks for it, running msgfmt --version and confirming the output is GNU
  gettext so a broken version-manager shim or an unrelated command named
  msgfmt cannot pass as installed. The fix hint covers both Homebrew
  (brew install gettext && brew link --force gettext) and a brew-free path
  (nix profile install nixpkgs#gettext).
• The Metal Toolchain check produced a false negative from a stale xcrun
  cache. After xcodebuild -downloadComponent MetalToolchain (a cryptex
  mount, not /Library/Developer/Toolchains/), xcrun metal --version still
  reported the toolchain missing because xcrun cached the earlier negative
  lookup. Doctor now retries the probe after xcrun --kill-cache when the
  first attempt fails, so a stale cache no longer masquerades as a missing
  toolchain while still correctly reporting a genuinely missing one.

Type of change

• Bug fix (the linked issue is a bug report)
• Feature (the linked issue is a feature request marked ready)
• Documentation
• Other (please describe)

How was this tested?

Shell-only tooling change (no Swift/app surface), so verification was against
scripts/doctor.sh directly:

• bash -n scripts/doctor.sh clean; ./scripts/doctor.sh exits 0 on a
  correctly provisioned machine (all checks green).

• msgfmt check exercised with PATH shims: real GNU msgfmt -> pass; a shim
  exiting non-zero, a non-GNU command named msgfmt, and an absent binary
  each -> doctor turns red with the actionable fix.

• make check passes (format + lint)

• make test passes

• I built and ran the app to confirm the change works

Checklist

• This pull request is linked to an issue with Closes # above.
• For a feature, the linked issue is labeled ready.
• I am the author of this work and accountable for it; no commit is authored or co-authored by an AI agent.
• I have read the Contributing guide and the Code of Conduct.