Plan 05 closes: src/background/index.ts is now a pure coordinator with
zero video-buffer state, T-1-04 mitigations on both onConnect and
onMessage, OFFSCREEN_READY handshake, port-based buffer fetch via
'video-keepalive' port, IDB orphan cleanup on install, and chrome.offscreen.hasDocument()
re-sync on SW respawn (audit P1 #8). 9/9 vitest tests still green;
tsc clean; no as any / @ts-ignore.
REQ-video-ring-buffer stays pending — Plan 07's ffprobe gate owns the
final completion marker.
