feat: metadata

[hampsterx]

Description

• Adds optional JSON metadata generation alongside golden PNG files, providing exact pixel bounds and semantics
  tree data for each scenario
• Two new config flags: metadataEnabled (bounds) and semanticsEnabled (accessibility tree) — both default
  to false, fully backwards-compatible
• Addresses upstream issue #171 (semantics testing)

Motivation

Golden tests output PNG images with scenarios in a grid, but row heights vary and there's no programmatic way
to query scenario positions or accessibility data. This makes reliable cropping require OpenCV or manual
calculation (8-50px error). The metadata JSON gives exact coordinates from Flutter's layout engine and
optionally the semantics tree for accessibility testing.

Key decisions

• Config lives on AlchemistConfig, not per-variant GoldensConfig — metadata is orthogonal to rendering
  options, and JSON path derives from goldenPath (already per-variant)
• Captures before whilePerforming — bounds are stable regardless of interaction, semantics reflect
  resting state
• Coordinates are logical pixels at devicePixelRatio 1.0 — maps directly to PNG pixels, no scaling needed

Type of Change

• [x ] ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore