[hcallahan-lowrisc]

Description

A sim cfg's primary_build_mode decides whose coverage goes first in the merge.
If a run selects only tests that use other build modes, and more than one of them, dvsim stops with primary_build_mode ... is invalid, even though nothing else in the run depends on that build.
So a cfg whose default build is not the primary one can't be run without editing the cfg, and neither can a run of only the tests that need special builds.

dvsim now uses the first of the run's builds by name instead, rather than whichever build the test order happens to give first.
It warns if coverage is enabled; without coverage, nothing reads the primary build mode.

What stays the same:

• A run that builds the primary mode is unchanged.
• A primary_build_mode that names no build mode of the cfg is still an error.
• A run with a single build uses it, and one that follows an equivalent build follows it, as before.

Compatibility

Runs that worked before are unchanged; a run that used to stop with this error now runs.

Testing

• pytest: 506 passed (499 on master), which adds seven cases in tests/sim/test_build_modes.py: the mode is kept when built, a single build becomes the primary, an equivalent build is followed, the fallback with and without coverage, an undefined primary, and a run with nothing to do.
  ruff check is clean.

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).