Phase C parity (ADR 0003): restore the legacy startProxy off-air behaviour
for daemon-served proxy streams. After registering a proxy source, live.php
calls FanoutClient::probe($id, on_demand_wait_time*1000) → daemon
POST /probe/<id>, which prewarms the puller and waits for the source to
produce data. If it stays dead, live.php shows the not-on-air page instead
of X-Accel'ing the viewer onto an empty/hanging daemon stream. A warm stream
(already producing) returns immediately, so only a cold/dead source pays the
wait — same as the legacy path.
Non-proxy off-air was already covered (the process block waits for the
playlist, and A3 gates on isStreamFed/has_data).
Box-validated: dead-source proxy → not-on-air video after on_demand_wait_time
(20s here); live-source proxy → fast play via the daemon (4.4 MB valid TS).
PHPStan clean, 11 FanoutClient tests.
Refs ADR 0003 (Phase C). Daemon: XC_VM_Binaries 0.4.0.
Divarion_D
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