feat(signals): slop signal — trivial/whitespace churn

[JSONbored]

Parent roadmap: #525
Parent epic: #530

Goal

Implement the trivial / whitespace-only churn signal.

Reuse

buildLocalScoreInput token split + isCodeFile/isTestFile (local-branch.ts).

Acceptance

High-churn / ~0 net-source diffs raise the signal.

Testing

npm run test:ci, 97%+ coverage. Public/private output boundary preserved; deterministic where applicable.