Implement automatic nightly batch isolation and regression bisecting

[litroc]

Context

Implements LI-123, MLX-100, and REL-30.

Scope

Isolate failing dependency updates in multi-change candidates while allowing independently proven green candidates to continue toward release.

Acceptance criteria

• A negative multi-update fixture identifies the responsible change.
• Dependency-graph isolation and controlled bisecting are reproducible.
• Independently green updates continue without unsafe partial promotion.
• Blocked updates receive cause, matrix, evidence, and owner.
• A corrected Renovate version is admitted to a later nightly.
• Completion evidence is linked here and to LI-123.