mfc.sh: add --prebuilt-prefix for running against prebuilt binaries

[sbryngelson]

Adds support for running mfc.sh run and mfc.sh test against an externally-installed MFC (e.g. a Spack install prefix) instead of rebuilding from source.

Motivation

A separate Spack package for MFC is being prepared. Without first-class support in MFC for "use my prebuilt binaries from /bin", the Spack recipe has to ship a ~100-line shell wrapper that fakes an MFC working tree via cache-dir symlinks and aspirational env vars (MFC_PREBUILT_BIN_DIR and friends) that no MFC code actually reads. Reviewers on the Spack side will (rightly) push back on that. This change moves the "find installed binaries + skip the build steps" logic where it belongs: inside MFC.

Changes

• New CLI flag --prebuilt-prefix=<PATH> on mfc.sh run and mfc.sh test. Also settable via MFC_PREBUILT_PREFIX environment variable (CLI wins).
• When set:
  • Target.is_buildable() returns False for every target (implies --no-build).
  • Target.get_install_binpath() resolves to <prefix>/bin/<target> instead of <cwd>/build/install/<slug>/bin/<target>.
  • toolchain/bootstrap/python.sh skips venv creation; caller is responsible for putting python3 + toolchain deps + the mfc package on PYTHONPATH.
  • mfc.sh's trailing deactivate call is now guarded (no-op when there's no venv to deactivate).
• Zero behavior change when the env var is unset and the flag is absent.

Testing

Local:

• ./mfc.sh format -j 8 clean
• ./mfc.sh precheck -j 8 6/6 pass (formatting, spelling, toolchain lint, source lint, doc refs, parameter docs)
• --prebuilt-prefix shows in both run --help and test --help
• get_prebuilt_prefix() precedence verified: CLI > env > None, empty-string CLI falls back to env
• MFC_PREBUILT_PREFIX=/tmp/fake ./mfc.sh run --help runs cleanly, skips venv

End-to-end MFC build + test verification is what this PR's CI will cover (local build blocked by an unrelated apple-clang 21 + ncurses toolchain issue).

Follow-up

After this merges, the planned spack-packages PR for mfc collapses from ~360 lines (with embedded shell wrapper) to ~120 lines and just sets MFC_PREBUILT_PREFIX=<prefix> in setup_run_environment.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.31%. Comparing base (7081882) to head (0d6efe1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1444   +/-   ##
=======================================
  Coverage   61.31%   61.31%           
=======================================
  Files          72       72           
  Lines       19771    19771           
  Branches     2852     2852           
=======================================
  Hits        12123    12123           
  Misses       5699     5699           
  Partials     1949     1949           

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