Phase 1 of extracting the live ffmpeg command assembly out of startStream.
buildLive(array $data): string is a byte-faithful copy of the inline assembly,
fed from a prepared $data array instead of loop-local state, with the delay
playlist I/O and segment-start/sleep left in startStream (arriving via
$data['segmentStart']/['delayActive']). The one non-verbatim change is the
ac3/eac3 dts_legacy bin switch, now a local reassignment instead of mutating the
$rFFMPEG_CPU/$rFFPROBE globals (the $rFFPROBE write was already dead). Homed on
StreamProcess for now so the private output/logo/aac helpers resolve via self::;
a FFmpegCommand facade is a follow-up.
startStream computes buildLive() alongside the inline assembly and error_log()s
any divergence, but still executes the inline $rFFMPEG -- shadow mode, no flip.
Once the diff log stays empty on real traffic the call site can switch over.
Characterisation tests cover simple/custom_ffmpeg/loopback/delay/rtmp branches
and assert no unresolved {TOKEN} survives.
Verified: phpstan level 5 green, phpunit 397/397, make gates green.
