feat: bug fixes, widget elements, deferred dithering, pixel code sizing

[eigger]

Fixes

• plot: default y_end derived from y_start (was x_start) — wrong vertical extent
• diagram: accept float bar values; clear error on non-positive max; require x/y/height
• plot: stop mutating the caller's history list in place
• dlimg: use startswith for url scheme detection (was substring in)
• group: wrap child handler errors with element context, like the top loop
• colors: support #RGB shorthand; warn (not silent white) on unknown color names

New elements (widgets.py)

• legend: color-swatch <-> label rows (the companion pie/plot lacked)
• star_rating: full/half/empty stars
• battery: vector battery gauge with proportional fill + low-color threshold

Dithering model

• Quantization is now deferred: elements draw in full color and the whole image is mapped to the palette once at the end; dither picks halftone vs flat. Output is always strictly on-palette; in-palette colors stay crisp.
• Per-element dither: true/false override on any element (isolated + quantized at draw time), shared via a new dispatch module so group children get it too.

Code element sizing (predictable, pixel-based)

• barcode: width/height scale to a pixel box (was mm + DPI only); expose dpi; route colors through the palette
• qrcode/datamatrix: width/height fit to a square pixel box (crisp, integer upscale)

Docs/examples/tests

• README: deferred-mapping + per-element dither sections, ESL(3-color)/label(2-color) dithering samples, widget + code-sizing reference updates
• new example generators (dither labels) and element previews
• tests: 97 passing (deferred/per-element dither, widgets, code pixel sizing, fixed-bug regressions)