COLRv1 group rendering, zero-callback layered glyph cache, WebGPU blend parity and CPU-exact brush colors

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Consumes the reworked Fonts renderer API and completes two WebGPU parity efforts.

COLRv1 groups

BaseGlyphBuilder and RichTextGlyphRenderer implement BeginGroup/EndGroup; the canvas
lowers each group to one isolated layer whose modes apply when it ends. Caller opacity
applies once to the outermost group, never per leaf. The per-glyph clip bounds become a
single rectangle intersect on each operation's rasterizer interest; no polygon clipping is
involved anywhere.

Layered glyph cache

A layered COLR cache hit now replays its stored operation sequence with zero renderer
callbacks: no outline decode and no path construction per draw. Replay tests assert
operation-for-operation equality and pixel-identical output.

WebGPU blend parity

blend.wgsl ports ImageSharp's AssociatedAlphaPorterDuffFunctions and the straight-RGB
blend curves for every PixelColorBlendingMode. Stores quantize in-shader
(round-to-nearest-even binary16, exact unorm packing) so both backends store identical
pixels.

CPU-exact brush colors

Solid colors and gradient ramp texels snap through ToPixel<TPixel> at flush, mirroring the
CPU's render-time brush creation: the retained scene streams specialize once per pixel
format and ride the ordinary uploads. Opaque fills are bit-exact against the CPU;
translucent fills sit within the wire format's binary16 rounding, documented at the patch
sites. Also fixes stale recolor payloads surviving a pixel-format switch.

[codecov]

Codecov Report

❌ Patch coverage is 92.10526% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 79%. Comparing base (1f6ad7e) to head (c3d5012).

Files with missing lines	Patch %	Lines
...eSharp.Drawing/Processing/RichTextGlyphRenderer.cs	85%	17 Missing and 5 partials ⚠️
...rc/ImageSharp.Drawing.WebGPU/WebGPUSceneEncoder.cs	95%	4 Missing and 2 partials ⚠️
src/ImageSharp.Drawing/Text/BaseGlyphBuilder.cs	86%	3 Missing ⚠️
...eSharp.Drawing/Processing/DrawingCanvas{TPixel}.cs	96%	2 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #412 +/- ##
======================================
Coverage 79% 79% ======================================
Files 232 232 Lines 27005 27340 +335 Branches 3086 3112 +26 ======================================
+ Hits 21541 21858 +317 - Misses 4574 4589 +15 - Partials 890 893 +3 

Flag	Coverage Δ
unittests	79% <92%> (+<1%)	⬆️

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