feat: Golden image tests for widget rendering

[kolkov]

Part of gogpu/gogpu#373 — Headless Golden Image Testing (ADR-053), Phase 4.

Summary

Golden image tests for widget rendering — automated snapshot testing across all themes and widget states. Uses gogpu golden test harness (Phase 2) with headless software backend.

Part of the ecosystem-wide golden image testing initiative (gogpu ADR-053).

Test Coverage

Core Widgets

• Button, TextField, Checkbox, Radio, Switch, Slider
• DropDown, ComboBox, ProgressBar, Spinner

Complex Widgets

• ListView (scroll, selection, row lifecycle)
• GridView (cell layout, selection)
• TreeView (expand/collapse, nested)
• TabBar (tab switching, overflow)

Themes (same widget × 4 goldens)

• Material 3
• Fluent
• Cupertino
• DevTools

Widget States

• Default, hover, pressed, focused, disabled
• Error state (TextField validation)
• Selected/unselected (Checkbox, Radio)

Layout

• Padding, alignment, nested containers
• Responsive layout at different window sizes
• HiDPI scaling verification

Damage Pipeline

• Verify partial repaint produces same result as full repaint
• Widget dirty flag correctness

Regression Targets

• Issues ListView: dropped row decorators never release their cached GPU scene — peak heap grows with scroll distance #173-182: ListView lifecycle bugs
• Window resize releases ALL boundary textures and force-dirties every widget — full-tree GPU texture recreation burst on every resize tick #176: resize full-tree texture recreation
• Theme consistency across platforms
• Damage pipeline correctness

Blocked By

• feat: Golden image tests for 2D rendering gg#432 — 2D rendering goldens (Phase 3)
• feat: Golden image test harness (headless rendering) gogpu#374 — golden test harness (Phase 2)
• feat: Headless software surface for golden image testing wgpu#256 — headless software surface (Phase 1)