[L04] Runtime contention

[yunseo-kim]

Scope

Run controlled contention across npm, publisher, and manifest mutation jobs with the same repository/ref group.

Contracts

ADRs 0066, 0067, 0074, 0075

Dependencies

L01-L03

Blocking issues: [L01] #39; [L02] #40; [L03] #41.

Executable QA

`gh workflow run cross-surface-contention.yml --repo "$CONFORMANCE_REPO" -f release_tag=refs/tags/v0.0.0-contention`; `gh run watch "$RUN_ID" --repo "$CONFORMANCE_REPO" --exit-status` must exit 0. Downloaded timeline must show no overlapping mutation intervals, cancellable pre-mutation jobs, segment-entry revalidation after wait, same-run convergence, and foreign rejection for a new run ID.