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ID:WORK-428Status:ready
bg guest body + engine relocation + bare-surface guardrail
The gate for SPEC-104: teach bg to host a live sandbox as a bare, presentational, full-bleed backdrop. Covers §1–§4 — the constrained body, the engine relocation that mirrors bg-video, the bare-surface guardrail, and boot-frame layering. Everything downstream (the preset, the showcase) builds on the data-bg-guest contract this establishes.
Constrained body (§1) — bg (today a directive with no body, packages/runes/src/tags/bg.ts) gains an optional body holding a single presentational guest. bg transforms it normally (so the real sandbox rune runs with file resolution + sanitisation via config.variables.__sandboxReadFile), tags the rendered output data-bg-guest, forces height="fill" (the SPEC-101 host-owned-height mode), and marks it presentational. The tagged element is emitted alongside the existing bg-* metas.
Engine relocation (§2) — engine.ts §1f, when it raises the bg layer, collects any data-bg-guest descendant into the data-name="bg" div — above the --bg-image boot frame, below overlay/scrim, a sibling of the bg-video branch — and marks it consumed so it is not also rendered in content flow. The SPEC-090presentational demotion applies (it is in the bg layer).
Bare-surface guardrail (§3) — the body accepts a presentational guest only; a chromed content rune (video/audio/figure) emits a SPEC-084-style build warning redirecting to the media-guest slot or bg video="…". Bare image/video backdrops keep their existing bg src=/video= attribute path, unchanged.
Boot-frame composition (§4) — a --bg-image (image/gradient) paints behind the guest; overlay/scrim above it; a host's positioned image still flows to og:image / structured-data image via the host's normal media extraction.
Acceptance Criteria
bg accepts an optional body holding one presentational guest (sandbox); bg transforms it (real sandbox rune — file resolution + sanitisation), tags it data-bg-guest, and forces height="fill" + presentational posture.
The engine (§1f) relocates a data-bg-guest element into the data-name="bg" layer — above the --bg-image boot frame, below overlay/scrim, sibling to bg-video — and marks it consumed so it does not render in content flow; SPEC-090 demotion applies.
A chromed content rune (video/audio/figure) in the bg body produces a build warning redirecting to the media-guest slot or bg video="…"; bare image/video backdrops keep their existing attribute path (unchanged).
Boot frame composes: a gradient/image paints behind the guest; overlay/scrim above it; a host's positioned image still flows to og:image / structured-data image.
Unit tests cover the body transform + tagging, the engine relocation + consumption, the guardrail warning, and the boot-frame layering; contracts regenerated.
Dependencies
None within the milestone — this is the SPEC-104 gate. WORK-429 (preset) and WORK-430 (docs) depend on the data-bg-guest contract here.