refactor(coverage): give the changed-lines diff one owner and pass it on stdin

[thymikee]

Summary

parseUnifiedDiff, ChangedFileDiff, and normalizePath move from scripts/coverage-changed/model.ts to scripts/lib (repo-git.ts beside readChangedLinesDiff, the diff they parse; repo-path.ts for the path rule), with their tests moving alongside (scripts/lib/repo-git.test.ts, repo-path.test.ts, run by the coverage-model gate). The two changed-line gates stay separate — only coverage-changed has a waiver label — but neither owns the diff format anymore.

check:coverage-crap now pipes the diff to fallow health --diff-stdin (fallow 3.32; documented as equivalent to --diff-file -) instead of writing coverage/fallow-crap.diff. No artifact of an earlier run can supply a scope, and run.test.ts seeds a stale artifact at that path with a debt-charging scope and proves the run ignores it and leaves it untouched.

11 files, +174/−158. No behavior change; scope is exactly the two audit items.

Validation

Head 3b208bfb0. pnpm check:affected --run passed on the head under the gate lock. Before/after proof for the stdin swap on a real git repo + real fallow 3.32 (fixture shaped like run.test.ts, edited debt function): reports and exit codes byte-identical (diff clean, both exit 1 with the CRAP 56.0 finding); after the change the run writes only fallow-crap-coverage.json under coverage/. Moved parser/tests are byte-identical (git diff -M90% + byte-compare); both test suites (23 + 59) green at their new paths. format, lint, typecheck, check:gate-manifest green. CI owns the instrumented Coverage and CRAP verdicts on the PR head.

[cubic-dev-ai]

All reported issues were addressed across 11 files

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Size Report

Metric	Base	Current	Diff
Installed (including dependencies)	5.17 MB	5.17 MB	+59 B
Package (unpacked)	5.17 MB	5.17 MB	+59 B
Package (download)	1.55 MB	1.55 MB	+25 B

Startup median (7 runs, lower is better):

Scenario	Base	Current	Diff
CLI --version	28.8 ms	27.8 ms	-1.0 ms
CLI --help	81.2 ms	76.5 ms	-4.7 ms

[thymikee]

This PR is ready at 3b208bf. CI is green, with 26 checks and none failing. The coverage-changed and coverage-crap gates and their moved tests run in the Coverage CI job, and the package.json change adds those tests to the coverage-model and coverage-crap-model gates. There are no conflicts, and nothing else stands between this PR and merge. Not blocking, and you can take or leave both: the doc comment on parseUnifiedDiff at scripts/lib/repo-git.ts:85 explains why it sits beside its producer, and the module layout already shows that, so one line describing the function is enough; the comment at scripts/coverage-crap/run.test.ts:219 recalls what an earlier release wrote, and it could state only the rule that no file under coverage/ supplies or receives the scope. I did not run the script test suites or pnpm check:affected. My local fallow is 2.104.0, not the pinned ^3.32.0, so I could not confirm the empty-stdin behavior on 3.32 or reproduce the byte-identical before/after run described in the PR body. I also did not mutation-test the seeded-artifact regression, and I judged it by reading the old code, which wrote the file.