HSV filter layer + destructive action

[TheTechromancer]

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HSV/HSL/HSY adjustment filter + destructive-apply parity for all filters

Adds a Hue/Saturation adjustment filter (HSV, HSL, HSY luma-weighted, plus Colorize) and back-fills destructive apply for every parametric filter (Curves, Levels, HSV) through one generic, selection-aware path.

Engine / GPU (Rust)

• set_filter_params handler (engine/layers.rs) mirroring set_void_params, wiring the previously-dangling updateFilterParams call. add_filter now coerces params against the filter's schema instead of &[].
• ParamFilter substrate (gpu/param_filter.rs): the single-pass RGBA8 render half shared by all parametric filters — plain + masked (fs_* / fs_*_masked) pipelines, parameterized on whether the shader binds an aux texture ([src, aux, uniform] for the LUT family vs [src, uniform] for HSV). lut_filter.rs's LutFilter becomes the aux-carrying specialization lut_param_filter; curves.wgsl gains fs_curves_masked. Invert stays on the untouched MaskedFilterPipeline.
• Generic destructive apply (engine/filters/apply.rs, gpu/compositor.rs): dropped the params.is_empty() guard; apply_filter carries params and filter_node_region builds a real EffectCache via ensure(params, …) instead of EffectCache::empty(), so a destructive curves/levels/hsv honors both its params and the selection shape (in-shader select(orig, filtered, selected)).
• HSV filter (gpu/filters/hsv.rs, shaders/filters/hsv.wgsl): three colour models + colorize over one shader, packing five params into a 32-byte uniform. Colour-space conversions added to shaders/lib/colorspace.wgsl (HSL and HCY, Rec.601), ported from Krita's KoColorConversions. Deliberate deviation from Krita, documented in-shader: HSY caps chroma to gamut so luma is preserved exactly rather than clamping out-of-gamut RGB post-hoc.
• Node histogram (request_node_histogram handler, compositor::pump_node_histogram): bins a raster node's own texture so the destructive Levels modal still shows a histogram (there's no filter arm in the tree to intercept).

Frontend

• FilterParamsEditor.svelte: the param-editing surface (channel selector + Curve/Levels editor + scalar rows) extracted from FilterProperties.svelte, now with enum rows (EnumDropdown, gains a disabled prop) and the colorize-disables-model rule. Embedded by both the layer panel (live, undo-coalesced) and the modal (scratch params).
• FilterModal.svelte + state/filterModal.svelte.ts: the destructive-apply dialog, seeded from schema defaults, hosting the same editor. The Colors-menu action branches — param-free filters apply immediately, parametric ones open the modal.
• filterParams.ts gains options, seedScratchParams, filterParamMap, colorizeActive, cloneParamValue.
• README "Features & Roadmap": Hue/Saturation/Lightness checked off.

Tests

• HSV GPU (tests/filters.rs): identity is a no-op; red +120° is exact green in HSV/HSL but luma-preserving (not pure green) in HSY; full desaturation is achromatic across models; colorize preserves luma and ignores the model selector.
• Destructive-apply regression: destructive Curves/Levels/HSV over a rect selection change only the selected region (defends the mask + empty-cache bugs). The invert-on-R8-mask-with-selection shipping guard is unchanged and still green.
• Frontend (filterParams.test.ts): enum partitions to scalars with options; seedScratchParams deep-clones defaults; colorizeActive reflects the colorize bool.

[codecov]

Codecov Report

❌ Patch coverage is 84.00000% with 108 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/darkly/src/engine/filters/apply.rs	68.88%	56 Missing ⚠️
crates/darkly/src/gpu/compositor.rs	77.60%	28 Missing ⚠️
crates/darkly/src/gpu/param_filter.rs	95.74%	8 Missing ⚠️
crates/darkly/src/engine/layers.rs	0.00%	7 Missing ⚠️
...s/darkly/src/engine/protocol/handlers/histogram.rs	37.50%	5 Missing ⚠️
crates/darkly/src/engine/rendering.rs	33.33%	4 Missing ⚠️

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