own-language budget step charges master's foreign lines to a PR whose base.sha is stale #7489

The foreign-line budget step in .github/workflows/own-language.yml diffs github.event.pull_request.base.sha...github.event.pull_request.head.sha. GitHub does not move pull_request.base.sha when master moves; it stays at the base the PR last saw (here: the base at PR creation). When a PR then merges a newer master into its branch, the three-dot diff starts at the old base, and every foreign line master gained in between is charged to the PR.

Seen on #7425 (Part of #6063): run 37603940543 used BASE cfda070b2 for head dca980b6f, which already contains master up to ba161bba1. The step denied cli/t27b/src/codegen.rs, ir.rs and tests/differential.rs, all from #7398 (lane 3), not from the PR. Diffed against current master the same head is inside the budget (49 foreign lines). Locally, the same check_budget against the stale bases of #7418 and #7447 also denies, and against current master all four lane 5 PRs pass.

Workaround used: gh api -X PATCH repos/gHashTag/t27/pulls/N -f base=master refreshes base.sha to the current master tip; a label toggle then re-runs the workflow with the fresh payload.

Possible fix (the workflow is protected, so this needs the owner's label): after checkout, compute the base as git merge-base origin/master "$HEAD" (or fetch github.base_ref and diff origin/$BASE_REF...$HEAD) instead of trusting the event's base.sha. The path check in the same file reads $BASE:tools/policy/foreign-exceptions.txt and has the same staleness, so an exception block that landed on master after the PR opened is not seen either.

Part of #6063

Boundary

• .github/workflows/own-language.yml
• cli/t27b/src/codegen.rs
• tests/differential.rs
• tools/policy/foreign-exceptions.txt

User Scenarios

• Given a pull request whose base.sha is stale because master has advanced since the PR was created, When the own-language.yml workflow runs its foreign-line budget step, Then the step must compute the diff base at runtime using the current merge-base with origin/master (or origin/$BASE_REF) instead of trusting the event's base.sha, so that only foreign lines introduced by the PR itself are counted.
• Given a PR that merges a newer master into its branch, When the workflow reads tools/policy/foreign-exceptions.txt for the path check, Then it must read the file from the computed current base so that exception blocks added to master after the PR opened are honored.

Requirements

• FR-001: The workflow .github/workflows/own-language.yml MUST replace the three-dot diff github.event.pull_request.base.sha...github.event.pull_request.head.sha with a diff computed from git merge-base origin/master "$HEAD" (or origin/${{ github.base_ref }}...$HEAD) so the base reflects the current tip of the target branch.
• FR-002: The foreign-exceptions file read in the same workflow ($BASE:tools/policy/foreign-exceptions.txt) MUST use the computed current base instead of the stale event base.sha.

Success Criteria

• grep -q 'merge-base origin/master' .github/workflows/own-language.yml exits 0.
• grep -q 'merge-base origin/\${{ github.base_ref }}' .github/workflows/own-language.yml || grep -q 'origin/\${{ github.base_ref }}\.\.\.\$HEAD' .github/workflows/own-language.yml exits 0.
• The workflow run on a PR with stale base.sha where master gained foreign lines since PR creation reports foreign-line count ≤ budget for a head that is within budget against current master (exit 0 on the budget step).
• The path check for tools/policy/foreign-exceptions.txt in the workflow references the computed base variable (exit 0 on grep -q '\${COMPUTED_BASE:-.*}:tools/policy/foreign-exceptions.txt' .github/workflows/own-language.yml or equivalent).
• All four lane-5 PRs (e.g., specs: layernorm, wrapup-auto and ternary_mac_synth test what they claim (Closes #7404) #7418, t27b: a test local named for a module var (Closes #7668) #7447) pass the budget step when re-run against current master (4 PRs pass).