[major] feat(color)!: migrate ThemeProvider to ktsu.Semantics.Color (gamma fix)

[matt-edmondson]

Summary

Migrates ktsu.ThemeProvider off its in-house color types onto the new ktsu.Semantics.Color package (2.3.0), and fixes a long-standing sRGB-as-linear gamma bug in the process. Breaking change → major version bump ([major]).

The public currency type PerceptualColor becomes ktsu.Semantics.Color.Color (linear RGB + alpha) across ISemanticTheme, SemanticColorMapper, the 38 themes, and ImGuiPaletteMapper.

The gamma fix

ThemeProvider previously loaded sRGB hex bytes as if they were linear (no decode) and fed them straight into the Oklab matrix and WCAG luminance. Now hex is parsed with Color.FromHex (correct sRGB→linear) and emitted to ImGui via ToSrgbVector4():

• Base theme colors render identically (hex→linear→sRGB round-trips — pinned by a new round-trip test).
• The semantic mapper's derived colors and accessibility/contrast numbers are now computed correctly (the intended behavior change).

What changed (5 commits)

1. Core library — deleted RgbColor/SRgbColor/OklabColor/PerceptualColor/ColorMath/AccessibilityLevel; rewrote SemanticColorMapper + ColorRange onto Color (algorithm preserved verbatim — gamut chroma search, lightness bucketing, interpolation — only the color primitives swapped; Oklab-L precomputed in hot loops); migrated 38 themes (PerceptualColor.FromRgb → Color.FromHex).
2. ThemeProvider.ImGui — ImGuiPaletteMapper emits color.ToSrgbVector4() (consumer-side gamma fix).
3. Demo — migrated; 13 unsafe != default "not-found" sentinels replaced with a Color? return + is { } color pattern.
4. Tests (new project — the repo had none) — 15 tests: base-color round-trip invariant (display stability), semantic-mapper in-gamut + lightness-ordering sanity, accessibility (black/white contrast = 21, AdjustForContrast reaches AA).
5. ApiCompat baseline regen + docs (README/CLAUDE) + [major].

Verification

• Local builds were green across all target frameworks at each step (net8/9/10 + netstandard2.0/2.1, plus the inherited net5/6/7); dotnet build ThemeProvider.sln clean; 15/15 tests pass. CI on this PR is the authoritative full-branch build+test.
• Built task-by-task with per-task review; the core library migration (commit 29379bd) had a full review confirming the gamma rule is clean and the SemanticColorMapper algorithm is preserved line-by-line against the original.

Notes / follow-ups

• ApiCompat: the CompatibilitySuppressions.xml baselines were regenerated for the major break (removed public types). Package-validation/pack runs in CI on merge — if it flags the removed types, the baseline may need a follow-up regeneration with -p:GenerateCompatibilitySuppressionFile=true.
• Pre-existing minor (predates this PR): an unused priorityLevels local in SemanticColorMapper.
• Next in the consolidation: migrate ktsu.ImGui.Styler (ImGuiApp) to route its ImColor output through ktsu.ImGui.Color + this updated ThemeProvider, once both publish.

🤖 Generated with Claude Code

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