ADR-0014 The scrim occludes by alpha

#Context

ADR-0013 defines overlay as interposing a scrim counted as chrome. First implementation resolved the scrim as an opaque surface at the substrate-adjacent ramp step, and the receipts inverted the ADR's expectation in the dark themes: at 0 Lc against the substrate, the scrim occludes at near-zero budget cost, reproducing the cheap-elevation failure ADR-0013 exists to reject — while in the light themes the same mechanism costs 84 to 99 Lc over a full Scope. One stacking vocabulary was resolving as two economies, and an opaque substrate-colored scrim also hides the covered scene entirely, where an overlay's contract is to dim it.

#Decision

The scrim is a single alpha mechanism in all four themes: one scrim color role resolving near-black everywhere, composited over the covered scene at a scrim alpha token's opacity. The budget measures the rendered composite — the scrim's cost is the actual change it makes to the pixels beneath, which requires the engine to composite alpha layers when resolving what a region shows rather than skipping translucent surfaces. The covered scene remains visible, dimmed; occlusion is a statement about interactivity, made structurally by the overlay Scope, not a disappearance.

#Rejected options

Opaque scrim at the substrate-adjacent step (the first implementation). Cheapest to build and already emitted. Rejected on its own receipts: free occlusion in the dark themes, total content erasure in all themes, and two economies for one token.

Ramps extended below the substrate in dark themes. Symmetry on paper. Rejected because field-night's substrate sits near lightness zero — there is no below — and because ADR-004 defines the substrate as the origin deviation is measured from; a ramp below the origin redefines the metric to serve one component.

Per-theme mechanisms (alpha in dark themes, opaque lightness in light). Tunes each theme's cost. Rejected because two mechanisms double the fixture surface and the engine paths for one visual job, with no principle deciding which themes get which — the shape of decision this system refuses.

#Consequences

The engine composites alpha layers over their backdrop when resolving rendered values; the Phase 0 fixtures and themed baselines are unaffected as none carries translucent surfaces, and that non-movement is verified at implementation. Dark themes still spend less on a scrim than light themes because dimming a dark scene emits less light — the metric being honest, not an exploit; recorded, not corrected. The must-fail overlay fixture required by ADR-0013 is authored in interior-light, where scrim cost is maximal. The scrim alpha token joins the receipt discipline; ADR-0013's overlay definition is amended by this ADR, not edited.