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ID:WORK-381Status:ready
Sandbox lazy/poster activation
sandbox is eager today: the iframe is created and its dependencies load on page render (it has a height knob but no deferral). That's fine for a small demo, but a heavy sandbox — a three.js scene (WORK-382), a large framework demo — on a perf-sensitive page like the site index costs a chunk of load time we deliberately avoided (WORK-350 and WORK-380 both limited live iframes "to keep the landing page fast").
Add a deferred-activation capability so a heavy sandbox can headline a page without the load-time hit — useful for any heavy sandbox, not just three.js.
activation attribute:eager (default — unchanged behaviour) | visible (mount when scrolled into view, via IntersectionObserver) | click (mount on explicit user activation, for the heaviest cases).
poster attribute: a static image (or the rendered static fallback) shown in the iframe's place until activation. The rune emits the poster + data-activation; the iframe's src/srcdoc and dependency <link>/<script> tags are not created until activation, so nothing downloads early.
Deferred mount lives in @refrakt-md/behaviors (alongside the other progressive-enhancement behaviours); the engine/renderer emit the poster markup and data attributes, the behaviour does the swap.
prefers-reduced-motion: a non-eager sandbox does not auto-activate; the poster + an explicit activation control are shown (so motion-sensitive users opt in).
Keep eager the default — zero change for existing sandboxes.
Acceptance Criteria
sandbox accepts activation (eager default | visible | click) and a poster image URL.
A non-eager sandbox renders the poster and defers iframe creation and dependency loading until activated (in view, or clicked) — verified by no early network request for the deps.
Under prefers-reduced-motion, a non-eager sandbox does not auto-activate; the poster and an explicit activation control are shown.
Build/SSR stays green (poster is static markup; the iframe is still srcdoc once mounted) and eager sandboxes are byte-for-byte unchanged (no regression).
Documented on runes/sandbox.md with a visible+poster example, and the behaviour is registered/loaded like the other progressive-enhancement behaviours.