bg guest body + engine relocation + bare-surface guardrail The gate for SPEC-104: teach bg to host a live sandbox as a bare, full-bleed backdrop. Covers §1–§4 + §7 — the constrained body, the engine relocation that mirrors bg-video, the backdrop posture (mounted-but-inert), the bare-surface guardrail, boot-frame layering, and reduced-motion/off-screen gating. Everything downstream (the preset, the showcase) builds on the data-bg-guest contract this establishes.
high complex
sandbox bg preset (transform-resolved) SPEC-104 §5: make a reusable sandbox backdrop applicable by name (bg="midnight-waves") like any other preset, by adding a sandbox descriptor to BgPresetDefinition — resolved at transform time (not the identity engine, which has no file access), expanding into the WORK-428 data-bg-guest body.
high complex