compat-gen skips old fixture versions in CI and stops at the first failure #10754

Problem or motivation

The backward-compatibility harness (vortex-test/compat-gen) has three gaps:

• No workflow runs compat.py check --mode all on its own. compat-validation.yml defaults to last (inputs.mode || 'last'), and its only caller, publish-dry-runs.yml, passes last. So once a newer release publishes a fixture under the same name, CI stops reading the files of older releases. For example, after Tracking Issue: Fix Canonical Decimal #10701 lands, the old decimal forms that Add compat fixtures for old decimal forms #10753 adds exist only under the versions published before it.
• One failing fixture stops the whole check. check.rs returns early on the first fixture write error, and assert_arrays_eq! panics instead of recording a failure.
• The README ("Git worktree workflow") says that compat.py publish --git-ref <ref> builds the binary from a worktree at that ref. The script only uses the ref to pick the version (scripts/compat.py:394, :416) and always builds the current tree.

Proposed solution

• Run --mode all before each release, or on a schedule.
• Record failures per fixture, and report them together at the end.
• Either make --git-ref build from the ref, or correct the README.

Additional context

Found while planning #10701. Research notes: #10633.