VersionCheck: compare base branch instead of registry; drop @assert

[mtfishman]

Summary

VersionCheck previously pulled the latest registered version from ITensorRegistry and asserted registered_version < current_version. That has a design gap: between a substantive PR merging (with a version bump) and the new version being registered, any number of further substantive PRs pass without bumping the version because registered_version is still the old value.

Concrete example seen on ITensor/ITensorGaussianMPS.jl#24:

State	Registry	main Project.toml	PR Project.toml
Before #23	0.1.13	0.1.13	—
#23 merged (not yet registered)	0.1.13	0.1.14	—
#24 opened	0.1.13	0.1.14	0.1.14 (no bump)

Old check: 0.1.13 < 0.1.14 → pass ✅. Reviewer intuition: ❌, the PR didn't move the version forward.

Fix

Compare the current branch's Project.toml version against the same file on the PR's base branch. Answers the question reviewers actually have ("did this PR move the version forward relative to main"), removes the registration-timing dependency, and matches how General-registry tooling thinks about the check.

Implementation

• New Fetch base branch step (actions/checkout@v4 is shallow on PR head only; the base branch isn't otherwise available): git fetch --depth=1 origin "$GITHUB_BASE_REF".
• Drop julia-actions/julia-buildpkg from this workflow — the new check only needs TOML (stdlib), not registry resolution.
• Use TOML.parse + VersionNumber + a real if / error("…") rather than @assert, so the failure produces a clean Julia error rather than an AssertionError. The error message spells out the base ref and both versions for actionable feedback.
• Skip gracefully when the base Project.toml is missing (initial-commit edge case) or when neither file has a version field. Error if only the PR removed the version field.

The localregistry input becomes unused but is kept (with an updated description) for backward compatibility with existing callers in ITensorPkgSkeleton-generated workflows.

Test plan

• CI passes on this PR (substantive change, version field on both base and head — should report a clean pass message).
• Future PR opened that touches a substantive file but doesn't bump Project.toml version reports a clear failure message naming both versions.
• Workflow-only PRs (e.g. .github/**-only changes) still skip via the classify-pr action.

🤖 Generated with Claude Code