v4.0.0: honest CI baseline

Validated against basef + carmine via SHA-pinned MRs (2b048d7f).
Both consumer pipelines GREEN before tag.

Removed:
- verify-downstream (broken Tier 4 trigger pattern since v3.1.0)
- pyinfra-smoke, bootc-boot-smoke (never wired)
- pipeline-source rule branches added in v3.3.1

Changed:
- workflow.auto_cancel.on_new_commit: interruptible

Migration: bump consumer pin to v4.0.0. Drop pipeline-source workflow
rule from consumers (was only needed for verify-downstream).

This is the foundation. v4.1.0 wires the substrate-instance cascade
(basef -> carmine trigger + CVE-watch + AI triage).