[hcallahan-lowrisc]

Builds on #282.
Until it merges, please ignore the first commit, feat: add --cfg-extend…, which is #282's.
Only the last commit, feat: record the git revision…, is new here.

Warning

This changes the results JSON.
A run that uses --cfg-extend writes a new key, extensions, which released dvsim (v1.59.1 and earlier) can't load.
A run without --cfg-extend writes the same JSON as before.
Details are under "Results JSON" below.

Description

A report says which project commit its results came from, but not which version of a --cfg-extend file was merged in.
When the extension lives in its own repository, as it usually does, the results of the tests it adds can't be traced back to it.

When a cfg merges an extension, dvsim now reads the git revision of the checkout that holds the extension file.
That revision is:

• the origin URL, with any credentials stripped as for the project's own;
• the commit;
• whether the checkout is dirty, which includes the extension file itself being untracked or ignored.

The revision is reported in three places:

• JSON results: each cfg's extensions, in command-line order, with that revision (null outside git).
• Markdown block report: a "Cfg extensions" list with the revisions.
• HTML block report: a badge for each extension in the header, e.g. Extension: ext.hjson sha: 1a2b3c4 (dirty).

Files that an extension imports from elsewhere are not recorded.

Results JSON

The new key extensions goes in each flow's <flow>.json, and in each entry of flow_results in index.json:

"extensions": [
  {"path": "/path/to/ext.hjson",
   "revision": {"origin": "https://github.com/org/overlay.git", "commit": "…", "commit_short": "1a2b3c4", "dirty": false}}
]

• Without --cfg-extend: the key is left out, so the JSON is unchanged, and any reader that loads it today still does.
• With --cfg-extend: SimFlowResults and SimResultsSummary forbid unknown keys (extra="forbid"), so an older dvsim rejects the file.
  For example, v1.57.1's dvsim-admin report gen fails with 1 validation error for SimFlowResults: extensions: Extra inputs are not permitted.
  Readers that parse the JSON leniently are not affected.
• Release type: I've typed it feat, not feat!, because nothing that works today stops working.
  Only results of the new option are new, and today's dvsim reads every JSON it wrote before.
  If you'd rather treat "old readers reject new output" as breaking, I'll add a BREAKING CHANGE: footer.
• Not in this PR: the JSON has no schema version that a reader could check.
  Making the models ignore unknown keys, or adding a version, would stop this happening again.
  I'd like to leave that to its own PR, and can open one if you agree.

Testing

• pytest: 531 passed (499 on master), which adds tests/report/test_extensions.py and git revision cases in tests/utils/test_git.py.
  They cover the JSON with and without extensions, the Markdown and HTML output, and a round trip.
  ruff check is clean.
• v1.57.1's dvsim-admin report gen loads the JSON of a run without --cfg-extend from this branch, and rejects that of a run with it, on the one extensions key.

Checklist

• All commits are signed off (git commit -s), indicating acceptance of the CLA
• Commit messages follow the conventional commit format (<type>[(<scope>)][!]: <description>)
  • The commit type correctly reflects the semver impact of the change
  • Breaking changes are marked with ! or a BREAKING CHANGE: footer
• New behaviour is covered by tests

This PR includes AI-assisted content generated with Claude Code. All changes have been reviewed and are the responsibility of the listed author(s).