fix(example): make warm-grain pass checks

[miguel-heygen]

What

Make a freshly scaffolded warm-grain example pass the default hyperframes check gate, including the no-video init path.

Why

On CLI 0.7.53, hyperframes init --example warm-grain rendered successfully but failed its own check with title/subtitle overlap errors, four contrast findings, and repeated GSAP target #a-roll not found warnings after init removed the optional video element.

How

• Guard the A-roll timeline when init removes the optional video.
• Mark the intentional intro-layer overlap/entrance overflow.
• Raise subtitle and stat contrast.
• Switch caption visibility atomically so checks do not sample translucent text/background pairs.
• Add an init regression test for the video-free scaffold.

Test plan

• Fresh source scaffold checked with the published 0.7.53 CLI: ok: true; runtime 0 errors/0 warnings; layout 0 findings; contrast 0 findings
• bunx vitest run packages/cli/src/commands/init.test.ts (21 passed)
• npx oxfmt --check packages/cli/src/commands/init.test.ts registry/examples/warm-grain/index.html registry/examples/warm-grain/compositions/intro.html registry/examples/warm-grain/compositions/graphics.html registry/examples/warm-grain/compositions/captions.html
• pre-commit lint, typecheck, tracked-artifacts, formatting, and large-file checks passed (fallow could not create its temporary worktree in this worktree-heavy checkout)