#21403 ci: hand off cmux NEXT DEV builds immediately

The cmux NEXT DEV source workflow currently waits for the scheduled dispatcher to hand a completed source run to the publisher. That adds several minutes of avoidable latency and makes publication depend on cron delivery.

This adds a downstream handoff job after the exact Release artifact-producing job succeeds. It dispatches cmux-next-dev-daily.yml with the current source run ID, while the existing 15-minute dispatcher remains the retry/backstop. The handoff also uploads a 14-day JSON receipt with source SHA, source queue delay, compile duration, runner name, runner labels, and handoff timestamp.

Validation:

• python3 -u tests/test_cmux_next_dev_daily_dispatch.py — PASS
• git diff --check — PASS
• actionlint .github/workflows/cmux-next.yml — PASS

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}

---

Summary by cubic

The cmux NEXT DEV source workflow now hands a completed source run straight to the publisher instead of waiting for the scheduled dispatcher, cutting several minutes of publication latency.

• Adds a handoff job that runs after the DEV Release compile job succeeds and dispatches cmux-next-dev-daily.yml with the current source run ID; the existing 15-minute dispatcher remains as the retry/backstop.
• Uploads a 14-day JSON receipt with source SHA, queue delay, compile duration, runner name, runner labels, and handoff timestamp.
• Extends the dispatch test to assert the handoff job, dispatch command, and receipt fields exist.