fix(cli): warn when a WebM render silently drops its alpha channel [P2] (#2044)

* fix(cli): warn when a WebM render loses its requested alpha channel

HyperFrames always encodes WebM with an alpha-capable pixel format
(yuva420p), but some ffmpeg/libvpx builds silently emit opaque yuv420p
even when handed alpha input and -pix_fmt yuva420p. The render succeeds
and plays back fine, so the lost transparency is only discovered after
compositing (users report shipping a solid-black clip and colorkeying it
out by hand).

After a WebM render, best-effort ffprobe the output's pix_fmt; if it
lacks alpha, print a non-blocking warning that names the concrete remedy
(--format mov / ProRes 4444). Only WebM is checked (mp4 is intentionally
opaque; mov/png carry alpha through paths that don't hit libvpx-vp9), and
a failed probe stays silent rather than warning speculatively.

Pure decision (pixelFormatHasAlpha / webmAlphaAdvisory) unit-tested;
verified end-to-end that a transparent WebM render now surfaces the
warning while an MP4 render stays silent.

* fix(cli): key WebM alpha check on ALPHA_MODE tag, not pix_fmt (R1 blocker)

R1 (Rames/Via) correctly flagged the detection as ~100% false-positive on
working builds. libvpx-vp9 stores the alpha plane in a Matroska
BlockAdditional sidecar, so ffprobe ALWAYS reports pix_fmt=yuv420p for a
correct transparent WebM (per docs/guides/rendering.mdx #1823 and the
webm-concat-copy smoke test). The real signal is the stream-level
ALPHA_MODE=1 tag: a working encode writes it; a build that can't emit the
sidecar omits it and produces genuinely opaque output.

Re-cut the probe to read stream_tags=alpha_mode (JSON, case-insensitive) and
warn only when a probed WebM lacks ALPHA_MODE=1. Tests inverted accordingly
(alphaMode:true → silent; alphaMode:false → warn). Verified end-to-end: a
transparent webm render on an alpha-preserving build (ALPHA_MODE=1) now emits
0 warnings; previously it warned on every webm.

packages/cli/src/commands/render.ts

@@ -57,6 +57,7 @@ import { formatLintFindings } from "../utils/lintFormat.js";
 import { loadProducer } from "../utils/producer.js";
 import { c } from "../ui/colors.js";
 import { formatBytes, formatRenderSummaryDetail, errorBox } from "../ui/format.js";
+import { warnIfWebmAlphaDropped } from "../utils/webmAlphaCheck.js";
 import { renderProgress } from "../ui/progress.js";
 import {
   trackRenderComplete,
@@ -1373,6 +1374,7 @@ async function renderDocker(
   // threaded back here; the summary shows render time only (never a wrong video
   // length). Probe the output with ffprobe if a duration figure is wanted here.
   printRenderComplete(outputPath, elapsed, options.quiet);
+  warnIfWebmAlphaDropped(outputPath, options.format, options.quiet);
   if (options.exitAfterComplete) scheduleRenderProcessExit();
   return { renderTimeMs: elapsed };
 }
@@ -1475,6 +1477,7 @@ export async function renderLocal(
     job.perfSummary?.compositionDurationSeconds,
     job.perfSummary?.totalFrames,
   );
+  warnIfWebmAlphaDropped(outputPath, options.format, options.quiet);
   if (!options.skipFeedback) {
     await maybePromptRenderFeedback({
       renderDurationMs: elapsed,