fix(cli): warn when a WebM render silently drops its alpha channel [P2] #10948

Job	Run time
Detect changes	15s
Preflight (lint + format)	42s
regression-shards (shard-2, style-15-prod hdr-hlg-regression style-1-prod many-cuts vfr-screen-re...	24m 26s
regression-shards (shard-3, style-7-prod style-8-prod style-10-prod css-spinner-render-compat web...	19m 12s
regression-shards (shard-6, overlay-montage-prod style-12-prod chat missing-host-comp-id png-sequ...	20m 59s
regression-shards (shard-5, style-4-prod style-11-prod style-2-prod animejs-adapter typegpu-adapt...	20m 34s
regression-shards (shard-7, sub-composition-video style-18-prod raf-ball-render-compat font-varia...	23m 13s
regression-shards (shard-4, style-16-prod style-9-prod style-17-prod iframe-render-compat variabl...	21m 21s
regression-shards (shard-1, hdr-regression style-5-prod style-3-prod mov-prores)	18m 56s
regression-shards (shard-8, style-13-prod style-6-prod vignelli-stacking gsap-letters-render-comp...	18m 42s
regression	4s
	2h 48m 24s

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