Pattern Editor · Image tab: zoom-out fill level + alpha/gamma contrast controls

[mbreiser]

Two enhancements to the Pattern Editor Image tab (pattern_editor.html #imagePane, backed by js/pattern-editor/tools/image-converter.js).

1. Zoom out beyond the image, with a specified fill level

Today #imageScale (10–500%; ImageConverterState.scale clamped 0.1–5.0) maps the arena region onto the source image. When the arena region extends past the image bounds, the out-of-image area isn't well defined.

Request

• Allow scaling out further so the arena samples a region larger than the source image (the image occupies only part of the arena).
• Fill everything beyond the image edges at a user-specified level — 0–15 (GS16) / 0–1 (GS2). Add a "Fill level" control next to Scale; default 0 (dark).
• Likely: extend the Scale range and relax the 0.1 lower clamp; sample-out-of-bounds returns the fill level (respected by both GS2 and GS16 quantization).

Acceptance: load a small image, zoom out so it covers ~50% of the arena → surrounding pixels render at the chosen fill level; live preview + generated .pat match; GS2 and GS16 both honor it.

2. Contrast correction: alpha (gain) + gamma

Add simple pre-quantization tone controls in createGrayscaleSource / generatePattern:

• Alpha (contrast/gain), Gamma correction applied to the 0–255 grayscale before quantization to GS2/GS16.
• Two sliders with identity defaults (alpha 1.0, gamma 1.0 = no-op), live preview on #imagePreviewCanvas.

Acceptance: adjusting alpha/gamma updates the preview and the generated pattern; identity values reproduce current output.

Notes

• Consider MATLAB-host parity if there's an equivalent image path there.
• Reuse the existing #imagePreviewCanvas render for live feedback.

Related: Pattern Editor tracking #6.