Codex: define Video Lab profile contract for Memory Mesh and Lattice routing

[mdheller]

@codex Please take this workstream.

Context: Lattice QueryRoutingDryRunPlan now carries lab profile refs under a Slash Topics + New Hope + Memory Mesh governance envelope. Video Lab should define the video modality profile contract used by Memory Mesh and Lattice routing without launching video jobs in dry-run mode.

Scope:

1. Define a video lab profile shape for frame extraction, shot/scene segmentation, transcript alignment, visual embeddings, retention policy, and evidence refs.
2. Define dry-run profile selection behavior: no video reads, no model execution, no remote calls.
3. Map profile refs to future lab://video-lab/default and clarify how video complements current lab://image-lab/default and lab://vision-lab/default references.
4. Add fixture/schema validation.

Acceptance criteria:

• Repo-native contract/spec and fixture exist.
• Dry-run semantics are explicit.
• Contract can be referenced by Memory Mesh and Lattice routing without executing video runtime.