feat(script): scale-to-fit preview + DESIGN.md overlays named theme

[cuio]

Summary

Two fixes that together let projects render the way their DESIGN.md says they should — and let humans actually see what they rendered.

Preview scale-to-fit

Before: body was hardcoded to 1920×1080 pixels. In any viewport smaller than 1920×1080 (preview embeds, narrow browser windows, the studio's own iframe panel), only the upper-left chunk of the canvas was visible.

After:

• body is 100vw / 100vh with flex centering
• .hf-stage keeps native pixel dimensions but applies transform: scale(min(calc(100vw / Wpx), calc(100vh / Hpx))) so the whole canvas stays visible at any viewport
• The producer launches Puppeteer at viewport === canvas, so scale evaluates to 1 during real renders — no impact on captured output
• Both calc() operands carry px units so the division yields a unitless number (scale() rejects raw lengths)

DESIGN.md overlays the named theme instead of being skipped

Before: resolveProjectTokens short-circuited — if hyperframes.json had design.theme, it returned that theme and ignored DESIGN.md entirely. After hyperframes init that was the common case: every project shipped with a default theme name, and DESIGN.md was dead weight.

After: composition order is DEFAULT_TOKENS → named theme → DESIGN.md, with DESIGN.md overlaying only the roles its parser actually finds. So a project can pick data-drift-dark as a starting palette and tweak individual hex codes in DESIGN.md without losing the rest of the theme.

parseDesignBriefToTokens now takes an optional base parameter (defaults to DEFAULT_TOKENS) so callers can stack briefs on any starting tokens — backwards-compatible signature.

Test plan

• my-first-video (hyperframes.json says hackernoon-ft, DESIGN.md says deep black + electric purple + cyan) now resolves to bg=#0a0a0a, accent=#7c3aed, accent2=#06b6d4
• Reassembled the project — chart-scene renders the HyperSpeedrun "Data Drift meets Swiss Pulse" aesthetic (purple + amber timeline badges on deep black) instead of cream data-journalism
• Resized preview viewport to 1280×720 — transform: matrix(0.666667, ...) applied, full 1920×1080 canvas visible inside, body bg is dark
• Lint, format check, typecheck all pass

Notes

This compounds with PR #7 (atmospheres + transitions + kinetic templates) — once both land, the dark-themed projects will get the dramatic aurora/mesh atmospheres on their actual brand colours instead of the muted cream-multiplied versions.