Add cell-aware video processor contracts

[hansent]

Summary

Stacks on #2616 and adds the Phase-1 processor safety seam for multi-cell Video Sources plus an offline, deterministic two-cell staging validation harness.

• reads a bounded immutable VIDEO_PROC_CELL identity once per worker;
• asserts cell identity on claim/status/result calls;
• rejects wrong-cell and accidental cross-cell jobs before any execution side effect while preserving legacy jobs with no placement metadata;
• permits remote execution only when the job is explicitly marked remoteExecution=true;
• exposes bounded cell identity and rejection reasons in health/runtime/metrics;
• preserves cell identity through process-mode IPC;
• renders separate East/South benchmark Jobs and redacted evidence templates for assignment, connector, preview, placement, failure/recovery, and network proof;
• updates HANDOFF.md and the multi-cell RFC to reflect what is implemented versus still undeployed.

The harness is staging/render-only. It does not access Kubernetes, credentials, DNS, or the live capacity-test connector source.

Compatibility and dependencies

• Base/dependency: [POC — do not merge] Video sources: connector agent + warm processor + local media plane #2616 (hansent/video-poc).
• Legacy worker + legacy job remains accepted.
• Configured worker + legacy job remains accepted during migration.
• A placed job on a worker with no cell identity fails closed.
• This intentionally does not change the active L40S capacity/runtime settings.
• Deployment depends on the matching control-plane and staging-infra PRs; do not deploy independently.

Validation

• pytest -q tests/development/video_poc — 217 passed
• PYTHONPATH=. pytest -q development/video_poc/processor/test_*.py — 18 passed
• focused two-cell renderer dry run — 9 agents, separate East/South manifests, no inline secrets
• git diff --check

Whole-file Black checks for three touched POC files fail identically on the parent branch; this PR avoids unrelated formatter churn.

Deployment safety

Draft only. No staging or production deployment is included. Any staged failure injection, benchmark Jobs, or cell rollout requires a separate owner approval checkpoint.

[hansent]

Closing: this was stacked on the obsolete #2616 POC branch, and the processor files it modified were since rewritten in #2800 and extracted to https://github.com/roboflow/rf-video-processor. The multi-cell contracts (cell identity, wrong-cell claim rejection, explicit remoteExecution, generation fencing) will be reimplemented against the extracted runtime and the merged roboflow#14376 control plane; the design source of truth is docs/MULTI_CELL_SCALING_RFC.md in that repo. Useful test ideas from this branch will be carried over during that reimplementation.