fix(engine): merge colour per field, detect alpha correctly, defer PNG parse

Three defects in how ffprobe output and the PNG fallback are combined.

The cICP fallback was unreachable. `ffprobeColorSpace ?? stillImageMeta
?.colorSpace` discarded the PNG result whenever ffprobe returned ANY
colour field — and ffprobe emits color_space "gbr" for every PNG,
including a plain rgb24 with no colour metadata. So on the build the
parser exists for (reports gbr, does not decode cICP) an HDR PQ PNG
resolved colorTransfer "" , isHdrColorSpace() returned false, and the
still graded SDR. Now merged per field.

hasAlpha's anchor bound to one alternative. In
/(^|[^a-z])yuva|rgba|.../ the `|` is looser than concatenation, so
(^|[^a-z]) guarded `yuva` and nothing else. The list also omitted abgr,
ya8, ya16 and ayuv64, and `gray[a-z0-9]*a` matched only gray8a/gray16a —
names FFmpeg renamed to ya8/ya16 in 2013, so dead against modern builds.
A ya8 grayscale-plus-alpha PNG reported hasAlpha:false, resolveFrameFormat
picked jpg and the overlay flattened to an opaque rectangle. Replaced
with the start-anchored form studio-server already uses, extracted as
exported pixelFormatHasAlpha so the test asserts the shipped predicate
rather than a copy of the pattern.

The PNG parse ran eagerly and was discarded. It sat before the first
await, so readFileSync plus the CRC walk executed for every file before
a single ffprobe was spawned — a caller fanning out over
composition.images with Promise.all serialised entirely: 12 4K PNGs took
2649 ms against 170 ms probe-only, 2.5 s of event-loop stall that also
blocks Puppeteer IPC. On the happy path the value was then thrown away.
Now lazily memoized behind the paths that actually consult it.

Tests: 18 pix_fmt cases against the real predicate. Reverting the regex
fails 4.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/engine/src/utils/ffprobe.test.ts

@@ -3,7 +3,11 @@ import { EventEmitter } from "events";
 import { readFileSync } from "fs";
 import { basename, resolve } from "path";
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { extractMediaMetadata, extractPngMetadataFromBuffer } from "./ffprobe.js";
+import {
+  extractMediaMetadata,
+  extractPngMetadataFromBuffer,
+  pixelFormatHasAlpha,
+} from "./ffprobe.js";
 
 function crc32(buf: Buffer): number {
   let crc = 0xffffffff;
@@ -750,3 +754,24 @@ describe("crc32 works on every runtime the package declares", () => {
     expect(fresh.extractPngMetadataFromBuffer(png)).toBeNull();
   });
 });
+
+describe("pix_fmt alpha detection", () => {
+  // The old pattern's (^|[^a-z]) anchor bound to `yuva` alone, and the list
+  // omitted formats real files actually use.
+  const ALPHA = [
+    "yuva420p",
+    "rgba",
+    "argb",
+    "bgra",
+    "abgr",
+    "gbrap",
+    "ya8",
+    "ya16le",
+    "ayuv64le",
+    "yuva444p12le",
+  ];
+  const OPAQUE = ["yuv420p", "rgb24", "gray", "gbrp", "nv12", "yuv444p10le", "bgr0", "rgb0"];
+
+  it.each(ALPHA)("detects alpha in %s", (fmt) => expect(pixelFormatHasAlpha(fmt)).toBe(true));
+  it.each(OPAQUE)("reports %s as opaque", (fmt) => expect(pixelFormatHasAlpha(fmt)).toBe(false));
+});