buildSubtitleImport nested the metadata loop inside the import loop and reused
the outer counter $i. The inner loop advanced $i to count(files), so the outer
import loop exited after its first iteration: only the first subtitle was
imported (-sub_charenc -i), while -map/-metadata was still emitted for every
file — mapping ffmpeg inputs that were never added. Multi-subtitle movies got a
single imported track plus dangling -map references (broken output).
Split the nested loop into two siblings — import every file, then map every
file — and cache the count. All configured subtitles are now imported and
mapped to the correct input index.
Add tests/Unit/StreamProcessSubtitleTest.php covering empty input, a single
local subtitle, remote-server fetch, and the multi-subtitle case that
reproduces the defect (import count was 1, must equal N). Verified red→green
against the pre-fix code.
Verified: php -l clean, make gates green, phpunit 336/336 green.
