fix(cli): warn preflight when color grading meets no hardware GPU

[miga-heygen]

Summary

• check --browser-gpu (or default auto) on a composition using data-color-grading can blow past the navigation-ready timeout when the browser silently falls back to SwiftShader/software WebGL — the grading canvas's per-frame GPU readback has no fast path under software WebGL and has been measured at ~40x slower than an ungraded composition (vault finding color-grading-canvas-readpixels-gpu-stall-under-swiftshader.md, root-caused and live-reproduced by cli-repro). This is an inherent SwiftShader limitation (see graveyard swiftshader-de-speedup.md), not something to "fix" directly.
• Adds compositionUsesColorGrading() and detectColorGradingGpuStallRisk() (packages/cli/src/browser/gpuPolicy.ts): a cheap static HTML scan for data-color-grading, combined with the existing GPU-mode probe (forcing "auto" to get the ground-truth hardware/software answer even for an explicit --browser-gpu request, since that mode always reports back "hardware" verbatim otherwise).
• Wired into check's runBrowserCheck (packages/cli/src/utils/checkBrowser.ts): prints the warning before opening the browser (so it's visible even if navigation subsequently times out and the whole report gets replaced with a generic runtime failure — runCheckPipeline's catch discards runBrowserCheck's in-progress findings on that path), and also records it as a "warning"-severity runtime finding so a run that succeeds anyway still surfaces it in check --strict / check --json.
• Warning text suggests concrete next steps: a much larger --timeout, running on a host with a real GPU, or the documented monochrome-preprocessing workaround (grading intensity: 0) to skip the expensive per-frame pass entirely. Does not attempt to auto-scale the timeout or otherwise change render behavior — the underlying slowness is a genuine SwiftShader constraint, not a bug to paper over.
• requestedMode: "software" (deliberate --no-browser-gpu) is not warned about — the user already opted into the slow path knowingly.

PRINFRA-685

Test plan

• bunx vitest run packages/cli/src/browser/gpuPolicy.test.ts packages/cli/src/browser/gpuPolicy.colorGradingStall.test.ts — 10/10 pass, including: detection on/off the color-grading attribute, warns only when composition+software-GPU combo holds, stays silent (and never probes) for ungraded compositions or explicit --no-browser-gpu, forces the "auto" ground-truth probe even for explicit --browser-gpu, and treats a probe failure as "nothing to warn about."
• bunx tsc --noEmit -p packages/cli clean
• bunx oxlint + bunx oxfmt --write clean (no unwanted reformatting)
• bunx fallow audit --base origin/main --fail-on-issues clean (0 issues, 4 changed files)
• Pre-commit hooks (lefthook: largefiles/tracked-artifacts/lint/format/fallow/typecheck/commitlint) all passed
• packages/cli/src/utils/checkBrowser.test.ts fails with Error: No such built-in module: node: both with and without this change — confirmed pre-existing/environmental via a clean origin/main re-run, unrelated to this fix

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