Gap 7: Backgrounds animation presets (#1096)

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 <[email protected]>

docs/gaps.md

@@ -63,7 +63,7 @@ are grouped by area; the issue tracker has the rest.
 | 4 | **No CI gate on pull requests** | `.github/workflows` has release, packaging-lint, FreeBSD and Windows ARM64 jobs only; `cargo xtask ci` runs locally | Regressions reach main unseen; 49 community PRs landed in one batch (#598) | 4–8 h | G3 · this file |
 | 5 | **Playback and scrubbing feel** | #595 stutter on cached frames, #304, #329 slow scrubbing, #554 / #504 / #335 audio late or crackling, #586 puppet lag | Preview is where motion designers spend their day | 25–50 h | G5 · [hardware-parity.md](hardware-parity.md) |
 | 6 | **Linux and Windows interaction bugs** | #491 multi-layer edits, #488 rename arrows, #493, #492, #467 shortcuts, #268 drag and drop, #320 workspaces not persisting, #302 docking, #613 Direct3D 12 compositor, #596 GPU not detected | Many reports come from Linux; some Windows GPUs fail to start the compositor | 25–45 h | G2 · [ui-parity.md](ui-parity.md) |
-| 7 | **Animation presets: 31 against 621** | After Effects ships 621 `.ffx` presets (Behaviors, Backgrounds, Shapes, Synthetics, Text, Transitions…); we ship 31 original presets (8 text-animator, 23 Transitions: wipes, clock and fan reveals, blinds, dissolves, card flips, CC wipes, fades; #757) and read only our `.ecpreset`. Missing: Behaviors, Backgrounds, Shapes, Synthetics, Image and more Text | Presets are how many designers start; tutorials assume them | 15–35 h (original presets; `.ffx` reading is out of clean-room scope) | G8 · [effects-parity.md](effects-parity.md) |
+| 7 | **Animation presets: 54 against 621** | After Effects ships 621 `.ffx` presets (Behaviors, Backgrounds, Shapes, Synthetics, Text, Transitions…); we ship 54 original presets (8 text-animator; 23 Transitions: wipes, clock and fan reveals, blinds, dissolves, card flips, CC wipes, fades, #757; 23 Backgrounds: drifting gradients, fog, cloud and smoke, plasma and magma, cells, hexagons, scrolling grids and checks, pulse rings, bokeh, light sweep, #1096, driven by expressions so they keep moving, not seamless loops) and read only our `.ecpreset`. Missing: Behaviors, Shapes, Synthetics, Image and more Text | Presets are how many designers start; tutorials assume them | 15–35 h (original presets; `.ffx` reading is out of clean-room scope) | G8 · [effects-parity.md](effects-parity.md) |
 | 8 | **Mask and shape bugs** | #510, #513, #527 (first Intersect/Darken mask), #509 Roto Brush size, #303 Mesh Warp, #553 pen tools on motion paths | Masks are core compositing | 10–20 h | G2 · [target-app-parity.md](target-app-parity.md) |
 | 9 | **Effect behaviour bugs** | #580, #526 Fractal reset, #514 Apply Color LUT, #463 Curl Noise, #369 Motion Tile output size, #473 Cryptomatte selection, #474 multi-layer EXR decoded twice, #534–#536 community fixes pending | Each one breaks a look someone built | 10–20 h | G1/G2 · [effects-parity.md](effects-parity.md) |
 | 10 | **Colour pipeline differences** | #494 32-bpc blend modes clamp and apply opacity differently; #495 Working Gamma 2.4 (in, also for float footage: #750); #340 ProRes 4444 tagged bt709 but sRGB-encoded; ACES 2.0 / OCIO 2.5 configs (26.5) missing; #546 / #547 viewer colour | Wrong colour in delivery is a client-visible failure | 15–30 h | G1 · [file-format-parity.md](file-format-parity.md) |