Problem: the built-in animation preset library had no Backgrounds category
(gap 7: 31 presets against After Effects' 621).
Fix: 23 original Backgrounds presets in crates/engine/presets/backgrounds.json,
built from our own generator, noise, colour and stylize effects: Soft Gradient
Drift, Aurora Veil, Slow Fog, Cloud Bank, Dark Smoke, Ember Haze, Magma Flow,
Ice Flow, Pastel Plasma, Cell Drift, Crystal Facets, Hex Mosaic, Mirrored Silk,
Water Caustics, Blueprint Grid, Checker Scroll, Scrolling Stripes, Pulse Rings,
Bokeh Drift, Spotlight Floor, Dawn Sky, Sweeping Light, Grainy Wash.
The preset format gains `expr` (an expression per property, so backgrounds keep
moving for the whole layer) and point parameters written as fractions of the
layer size. Effects & Presets lists them under * Animation Presets > Backgrounds.
Tests: catalogue uniqueness and categories, apply/undo/redo of every preset
(engine), render of every preset with the expression engine: finite,
non-flat, deterministic, moving (host), panel listing and apply (ui-egui).
Sidecar, ATTRIBUTION.md, gaps.md row 7 and effects-parity.md updated.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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