feat: add stack auto-layout container with Tailwind-like class shorthand

[eigger]

Add a flexbox-style stack container (aliases row/column) that packs child elements along an axis with gap, padding, justify, and cross-axis alignment — children need no coordinates, the stack measures and positions them. Drawing is still delegated to the normal element handlers, so every registered element works as a child and existing absolute-coordinate payloads are unaffected (purely additive, no breaking change).

• New imagespec.classutil.parse_class: optional Tailwind-like class string shorthand that desugars to the structured keys (explicit keys win). Only the layout subset is recognized; styling/color classes are ignored so colors and fonts stay on each element's own keys. Faithful Tailwind spacing scale (N x 4px), arbitrary values (gap-[10]/p-[3px]), fractions (mt-0.5), and negative margins (-mt-2) for fine nudges.
• Per-child layout via the child's class or a layout: sub-dict (grow, self-align, margin) — kept separate from drawing keys so e.g. a diagram's own margin is never mistaken for a layout margin.
• _blit helper composites tiles with clipping so negative margins / overflow are safe.

Docs:

• New self-contained docs/authoring.md LLM authoring guide (output contract, layout decision model, class shorthand, common pitfalls, device sizes, and worked examples — each verified by rendering), linked from the README.
• Fix invalid one-line multi-key YAML examples in the README.

Tests: stack packing/alignment/justify/grow/margin/padding/offset + class parser (scale, arbitrary, fractions, negative, unknown-token) + matrix samples for stack/row/column. 117 passing, ruff clean.