[Rejessx]

Closes #183

Summary

Adds native non-destructive Adjustment Layers (HSB, Color Balance, Curves, and Gradient Map) with full support for built-in layer masks, opacity, blend modes, clipping masks, stack reordering, live re-editing, undo/redo, and Merge Down / Flatten.

Changes

Engine (crates/engine)

• Added LayerAdjustment (Hsb, ColorBalance, Curves, GradientMap), AdjustmentGradientStop, and Layer::adjustment: Option<LayerAdjustment> with sanitized() validation and serialization in crates/engine/src/document.rs.
• Added Document::add_adjustment_layer, Document::set_adjustment, cleanup on Document::rasterize, and repair sanitization in Document::repaired.
• Updated Layer initializers in native.rs, psd.rs, and project.rs.
• Added unit test adjustment_layers_sanitize_serialize_and_rasterize.

GPU Compositing (crates/gpu)

• Extended LayerUniform in crates/gpu/src/shaders/composite.wgsl and crates/gpu/src/canvas.rs with an adjustment flag and added layer_weight(p) + fs_blend handling (mix(backdrop, blended, w)) so semi-transparent backdrops preserve alpha without premultiplied-over darkening.
• Implemented Canvas::composite_adjustment, run_adjustment_filter, adjustment_pass, and per-layer cached 256x1 RGBA8 LUT textures (adjustment_tables) for Curves and GradientMap.
• Updated plain(doc, id) cache partitioning and blend_layer_into so Merge Down and Flatten work with adjustment layers.
• Added headless GPU tests in crates/gpu/tests/layers.rs:
  • adjustment_layer_hsb_mask_opacity_and_live_edit
  • adjustment_layer_curves_and_merge_down

Core Session (crates/core)

• Added AdjustmentLayerData, CanvasSession::add_adjustment_layer, CanvasSession::adjust_layer_begin, live parameter updates in adjust_hsb / adjust_color_balance / adjust_curves / adjust_gradient_map, and undo/cancel handling in adjust_end.
• Added CanvasSession::adjustment_layer_start in crates/core/src/adjusting.rs to populate UI controls from saved layer parameters.
• Added LayerInfo::adjustment and LayerOption::EditAdjustment in crates/core/src/layers.rs.

Desktop & Android UI (desktop, android)

• Desktop (desktop/src/layers.rs, desktop/src/ui.rs): added adjustment layer creation options to the Layers + menu, Edit Adjustment option, ADJ thumbnail badge, and live panel opening.
• Android (CanvasViewModel.kt, LayersPanel.kt): added adjustment layer options to the + menu and LayerOption.EDIT_ADJUSTMENT handling.

Verification

• cargo fmt --all --check — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed (0 warnings)
• procraft-engine: 439 passed, 8 ignored, 0 failed
• procraft-gpu --test layers: 32 passed, 0 failed
• procraft-core: 849 passed, 16 ignored, 0 failed
• procraft-desktop: 105 passed, 2 ignored, 0 failed (test binary exited with STATUS_ACCESS_VIOLATION during Windows driver teardown after all tests passed)
• Android release APK (.\gradlew.bat --no-daemon :app:assembleRelease) — built (arm64-v8a + x86_64)