Thread viewport dimensions through style resolution so viewport units
resolve correctly: 1vw = 1% of viewport width, 1vh = 1% of viewport
height, vmin/vmax use the smaller/larger dimension.

For layout properties (width, height, margin, padding), percentages are
now preserved as LengthOrAuto::Percentage through style computation and
resolved during layout against the containing block width. Height
percentages resolve against viewport height. Per CSS spec, even vertical
margins and padding resolve against containing block width.

Changes:
- Add Percentage(f32) variant to LengthOrAuto
- Change ComputedStyle padding fields from f32 to LengthOrAuto
- Add viewport parameter to resolve_styles, threaded through resolution
- resolve_length_unit now computes vw/vh/vmin/vmax from viewport dims
- Layout resolves percentage width/height/margin/padding against
containing block; nested percentages compound correctly
- Store css_margin/css_padding/css_offsets on LayoutBox for deferred
resolution during layout
- 9 new tests covering all acceptance criteria

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Wire up visibility property through the layout and render pipeline:
- Add visibility field to LayoutBox, populated from ComputedStyle
- Renderer skips painting (background, borders, text, images) for
hidden/collapse elements but still recurses into children
- Children can override with visibility:visible per CSS spec
- display:none already worked (excluded from layout tree); verified

9 new tests: 5 layout tests (display:none exclusion, hidden preserves
space, inheritance, visible override, collapse behavior) and 4 render
tests (hidden not painted, visible child of hidden parent painted,
collapse not painted, display:none not in display list).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add BoxSizing enum and box_sizing field to ComputedStyle with CSS
parsing support. Layout engine now respects explicit CSS width/height
and adjusts content dimensions based on box-sizing mode:
- content-box (default): width/height apply to content area only
- border-box: width/height include padding and border

Content area clamps to 0 when padding+border exceeds specified size
in border-box mode. Property is not inherited per spec.

9 new tests covering both modes, clamping, non-inheritance, and height.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>