Add validation and execution for external fixture bundles

[Miko997]

What changed

• Added metriplane external validate <fixture> and metriplane external run <fixture> --out <run> for installed-package use.
• Added strict, versioned JSON validation and run summaries while keeping normal failures traceback-free.
• Reused the frozen external contract validator for manifest, checksum, session, mapping, normalization-report, domain-pack, and cross-artifact checks.
• Kept Atlas as the sole process/event/incident engine; the external layer only validates, resolves manifest-declared inputs, invokes run_atlas(), and verifies generated evidence and regression artifacts.
• Added fail-closed output-overlap, symlink, nonregular-file, unsafe operational-ID, and terminal/report-control handling.

Durable provenance

External runs write external_source_provenance.json with schema metriplane.external_source_provenance.v1. Atlas hashes and references it from atlas_manifest.json, renders a concise report section, and includes the same record in every incident evidence bundle under provenance/.

The durable evaluation command now records canonical <fixture> and <output> placeholders while preserving the command, flags, run ID, and overwrite state. Machine-local fixture and output paths remain available only in the immediate CLI/run summary.

The evaluation identity now optionally records:

• actual_metriplane_git_commit
• actual_metriplane_git_dirty
• actual_metriplane_git_describe

These values come from the existing get_git_info() implementation and its environment overrides. repo_root is never serialized, and unavailable Git metadata remains null rather than being fabricated.

The Atlas hook is optional. Ordinary Atlas manifests, reports, and evidence bundles omit every external-only artifact and field.

Availability

The user guide states that these commands are newer than published Metriplane v0.3.0. PyPI and conda-forge v0.3.0 do not contain them; until a later release includes the functionality, the documented development wheel is built from immutable implementation commit a901ea8d3be62355997c08e3030512e4129ee03c.

This PR does not bump the package version or create a release.

Compatibility

Existing atlas validate-pack, atlas run, demo, FrameStateModel 1.0, domain-pack, bundle verification, and regression paths remain unchanged. No source-format runtime dependency or source-specific condition was added. External Source Contract v1 is unchanged.

Verification

Final PR head: 419c2412832f8c37156a6cb45b0c6e35ebd776ac.

• Focused external, Atlas, bundle, and regression slice: 250 passed.
• Full local non-launcher suite: 981 passed, 2 skipped.
• Documentation front-door tests: 10 passed; the final workflow/docs regression slice: 15 passed.
• mkdocs build --strict: passed.
• Wheel/sdist build and twine check --strict: passed; package version remains 0.3.0.
• Fresh-venv wheel smoke built from the final head and run outside the checkout with METRIPLANE_GIT_COMMIT=419c2412832f8c37156a6cb45b0c6e35ebd776ac: passed. The run recorded that exact SHA, produced 4 frames, 5 events, 1 deviation, and 1 incident; evidence verification and generated regression passed; run and bundled provenance were byte-identical, path-free, and repo_root-free; the fixture fingerprint was unchanged.
• GitHub CI Linux Python 3.13 full suite: 1042 passed, 1 skipped.
• Required workflows at the final head all passed: CI #341, Release Gates #149, Documentation chore(deps): bump ossf/scorecard-action from 2.4.3 to 2.4.4 #28, CodeQL Add a publication edition of the commissioned first-use evaluation #66, and ClusterFuzzLite Require explicit overwrite for temporary Atlas outputs #12.
• Release Gates passed all four fresh-wheel jobs: Ubuntu and macOS on Python 3.12 and 3.13.
• Frozen schemas/metriplane.external_source_contract.v1.schema.json remains byte-identical, SHA-256 b5544012d7d98f1fdc8aed56192c33ac16f4acebd6694778ad682743482722c4.

Limits

A passing run establishes contract compliance, local byte integrity/agreement, and reproducible Atlas evaluation of the supplied normalized state and process rules. It does not authenticate a publisher, prove upstream conversion correctness, physical ground truth, realism, sensor accuracy, safety, production suitability, endorsement, or independent adoption. Referenced remote sources are never downloaded, and adapter declarations are never executed.