feat(themes): per-theme HTML+GSAP template overrides

[cuio]

Summary

Themes can now ship their own scene templates that the planner picks from and the assembler renders, alongside the built-ins. Closes the loop between design-system specs (already in docs/design-systems/) and the production renderer — Claude design ships JSX as reference, this PR lets the same theme ship the production HTML.

How a theme ships a template

docs/design-systems/<theme-id>/templates/
├── cold-open.html       ← required: the markup with {{prop}} substitution
├── cold-open.json       ← optional: metadata + propsSchema sidecar
├── manifesto.html
└── manifesto.json

Templates register with namespaced ids <theme-id>__<basename> (e.g. dreamspace__cold-open). Double-underscore so the id is safe inside CSS class selectors — a dot would be parsed as a class separator and silently break the template's scoped styles, which is the bug I hit on the first port and fixed before shipping.

Template engine

Tight Mustache subset (packages/core/src/script/themes/templateEngine.ts, ~100 LOC, no deps):

Syntax	Meaning
{{prop}}	HTML-escaped value lookup. Dot paths supported ({{tokens.colors.bg}}).
{{prop|raw}}	Unescaped — only for trusted markup.
{{prop|json}}	JSON.stringify-safe for embedding inside <script>.
{{#each items}}…{{this}}…{{/each}}	Iterate. Inside the body: {{this}}, {{this.label}}, {{@index}}.

Always-injected context: {{scene_id}}, {{template_id}}, {{duration}}, {{is_hook}}, {{audio_src}}, full {{tokens.*}} tree, plus aliases {{colors.*}}, {{fonts.*}}, {{motion.*}}.

Wiring

• LoadedTheme gains a templates: Template[] field; the disk loader populates it.
• New resolveTemplateRegistry(projectDir) returns [...BUILTIN_TEMPLATES, ...activeTheme.templates].
• planScript accepts opts.availableTemplates; when supplied, the planner's tool catalog includes theme templates and validators accept them.
• assembleMaster accepts opts.templates; scene render uses the right registry.
• Studio /script/plan and /script/generate thread the merged registry through both calls.

Example port: Dreamspace T1 Cold Open

docs/design-systems/dreamspace/templates/cold-open.html — full port from the JSX reference: 4 concentric SVG rings drawing in via stroke-dashoffset, cyan dot orbiting the highlight ring (continuous theta tween via onUpdate with finite repeat math so the deterministic capture engine can seek), mono eyebrow, gradient-clipped wordmark, per-word tagline stagger. Honors the dreamspace tokens + power4.out ease.

The metadata sidecar declares whenToUse, durationRange: { min: 4, max: 6 }, hookOnly: true, props schema, and exampleProps so the studio could render a preview.

Test plan

• loadThemeRegistry surfaces dreamspace.templates as [dreamspace__cold-open]
• resolveTemplateRegistry returns [...BUILTIN_TEMPLATES, dreamspace__cold-open] (9 → 10 templates after merge)
• Forced one scene to dreamspace__cold-open + props = exampleProps, reassembled, browser-verified: 4 rings draw, eyebrow + wordmark + 5 tagline words render with the dreamspace token palette
• Lint, format, typecheck pass
• Studio's SSR cache-watcher picks up template file changes without restart

How designers add new templates

1. Claude design generates new template JSX in the theme's reference set.
2. Port the JSX → HTML+GSAP, save as docs/design-systems/<theme>/templates/<name>.html.
3. Write the <name>.json sidecar describing when the planner should pick it.
4. The cache-watcher fires, the planner sees the new template on the next plan, the assembler renders it.

No code edits required. The framework's built-in templates remain as the universal safety net — themes only need to ship templates where they can offer a meaningfully better look than the built-ins.

Future work

• More Dreamspace ports (T2 prompt-to-app, T3 by-the-numbers, T4 stack-layers, T5 manifesto, T6 launch-card) — straightforward but each takes time.
• Auto-port from Remotion JSX via Babel-standalone analysis (eliminates manual porting). Big project — not in scope here.
• Live preview in the studio's theme picker showing the template's exampleProps rendered.