Pure type-extension step for the Option C architectural refactor of the
offscreen↔SW port lifecycle (.planning/debug/empty-archive-port-race.md).
PortMessage.requestId is optional so PING/PONG keep their no-payload
shape. REQUEST_BUFFER and BUFFER will populate it once the recorder
(offscreen) and getVideoBufferFromOffscreen (SW) are wired up in the
next commits. The narrowing remains structural — every consumer must
still validate `type` before accessing `requestId`.
PONG joins PING as a no-payload liveness signal. The SW echoes PONG on
PING, closing the health-probe loop the offscreen uses to detect a dead
port (replacing the 290 s pre-emptive setTimeout that was the proximate
cause of the H1 race window per the bisect).
No runtime change. 52 tests, 10 RED still RED (waiting for impl).
tsc --noEmit exit 0; type-safety grep clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Mark
c6e8101860