Make pull-request build checks compile the actual feature branch

[Exohayvan]

Summary

The Build Current Branch workflow reports success for feature branches whose names do not begin with minecraft/ or stardewvalley/, while both real build steps are skipped.

Evidence

• .github/workflows/build.yml gates Minecraft and Stardew build steps with startsWith(github.ref_name, 'minecraft/') and startsWith(github.ref_name, 'stardewvalley/').
• Current PR branches use names such as minecraft-fabric-26.1.x-fix-recipe_snapshot_concurrency.
• Live Actions inspection for PRs fix(neoforge): guard null recipe source details #6, fix(quilt): guard null recipe source details #9, fix(forge): support one JAR across Minecraft 1.20-1.20.4 #11, fix(neoforge): coordinate recipe load producers #12, fix(quilt): serialize recipe load snapshots #13, and fix(fabric): serialize recipe load snapshots #14 found successful Build Current Branch jobs where both actual build steps had conclusion skipped.

Impact

Pull requests can satisfy a green build check without compiling any source, allowing incompatible or uncompilable changes to appear validated.

Reproduction / verification

• Push a harmless compile failure to a representative feature branch in a test context.
• Verify the required PR check fails because the relevant loader target is compiled.

Proposed direction

Use pull_request metadata and base branch/target mapping rather than relying on the feature branch prefix. Fail the job when no target build is selected.

Acceptance criteria

• Every loader and Stardew PR compiles the intended target checkout.
• A job with no selected target fails instead of succeeding as a no-op.
• Checks use the tested PR SHA and expose the selected target in logs.
• Workflow tests cover slash and hyphen feature-branch names.

Audit provenance

Candidate #13 from the read-only repository audit performed on 2026-07-20. This is separate from existing issues #3, #4, #25, and #27 unless explicitly linked later.