Add a source-grounded Step Runner to Studio Deliverables #2489

Goal

Turn Studio Deliverables into a source-grounded Step Runner so a user can follow a saved Video Pack one step at a time.

Acceptance criteria

• Compile ordered steps from saved requirements and action items, or a linked SOP when those are absent, without inventing timestamps or verification evidence.
• Show Step 0 prerequisites, step navigation, video seeking and clip replay, progress notes, and a full RUNBOOK.md view/export.
• Scope Studio chat to the active saved-pack step and its evidence window. The server must resolve the step from the stored pack, reject mismatches, and keep one step's conversation out of another.
• Preserve initial #step-N links when a stored pack is selected automatically; clear the hash on an actual later pack switch.
• Retain valid actions when optional extractor timing is malformed, and keep runbook IDs within the chat schema bound.
• Use a source-bound auJzb1D-fag test fixture and pass the affected web tests, lint, type check, production build, and required hosted checks.

Implementing PR: #2441. Related Studio layout issue: #2331. This issue covers the Step Runner behavior specifically.