Bug Resolution: Auto-Solver CIELAB Chroma Scaling, Histogram Spacing & Highlight Clamp Logic (#16)

[everyday3d]

Bug Summary & Root Cause

On colorful source photos (e.g. sunsets, autumn foliage, vibrant landscapes), the auto-solver produced muted or washed-out filament palettes and allowed dark or low-TD green spools to bleed into upper highlight micro-layers.

1. Chroma Collapse: Natural conversion from RGB to CIELAB without chroma scaling caused saturated warm tones (oranges, golds, rich reds) to be matched against dull neutral midtones in CIEDE2000 distance.
2. Cluster Mud Clustering: K-Means clustering selected multiple similar neutral midtones (e.g. gray + light gray + earth brown) without angular hue spacing.
3. Highlight Bleed: Lack of physical clamp logic allowed low-TD green or dark spools to be assigned to the final top 4 micro-layers, contaminating bright canopies and specular highlights.

Reproduction

1. Feed a high-chroma autumn canopy or vibrant sunset photo into the auto-solver.
2. Run 4-color or 8-color palette solving against a standard multi-filament catalog.
3. Observe multiple gray/brown midtones selected and green spools assigned up to layer 23/24.

Resolution

• Perceptual Saturation Boost: Added scaleLabChroma (1.30x scalar multiplier to $a^$ and $b^$) with shadow noise protection ($L < 22, C < 6.0$) and skin tone / fur cap (1.08x for $20^\circ \le h \le 78^\circ$).
• Histogram Spacing Constraint: Added getHueAngularDistanceDeg ($25^\circ$ angular distance threshold) in K-Means cluster merging and $+50.0$ penalty for duplicate neutrals unless manualNeutralOverride is set.
• Upper-Layer Highlight Clamp: Added isLowTdGreenOrDarkPigment clamping low-TD green ($TD < 4.0\text{mm}$) and dark pigments ($L < 42$ or $TD < 2.0\text{mm}$) to $\le \text{totalLayers} - 4$ with $+5000.0$ penalty in coordinate descent.
• Verification: Verified with 14 test suites and 82 unit tests.
• Reference: Resolved in Issue #16 and commit 4097d72.