feat: add screenshot compression for baseline artifacts

[tom-bot-for-rana]

Problem

Baseline artifacts store full-page PNG screenshots which can be large (often 2-5 MB per route). This increases:

• GitHub Actions artifact storage costs
• Upload/download times for baseline artifacts
• CI runtime (artifact upload is often the slowest step)

Proposal

Add optional PNG compression to reduce screenshot file sizes with minimal visual impact.

Config

{
  "screenshots": {
    "compression": "none" | "fast" | "aggressive"
  }
}

Mode	Description	Typical savings
none (default)	No compression, original PNG output	0%
fast	Lossy compression at quality 0.8	~40-60%
aggressive	Lossy compression at quality 0.5	~70-80%

Implementation

Use pngquant (the de-facto standard for PNG compression) via its Node bindings or as a subprocess.

Implementation Plan

1. Config validation (lib/snapdrift-config.mjs)

• Add screenshots.compression field to config validation
• Validate: must be one of none, fast, aggressive
• Default: none (no change to existing behavior)

2. Capture flow (lib/capture-routes.mjs)

• After screenshot capture, apply compression based on config
• pngquant via subprocess: pngquant --quality=80-90 --speed=1 --output <out> <in> for fast mode
• pngquant --quality=50-70 --speed=1 --output <out> <in> for aggressive mode
• Handle missing pngquant gracefully: log a warning and skip compression

3. Dependency

• Add pngquant-bin as an optional dependency (or sharp which includes PNG compression)
• In the action YAML, install the dependency before capture

4. Type definitions

• Add screenshots?: { compression?: "none" | "fast" | "aggressive" } to VisualRegressionConfig

5. Tests

• capture-routes.test.js: test that compression mode is applied
• Test that missing pngquant logs a warning but doesn't fail
• Test that compression: none produces identical output (no regression)

6. Documentation

• docs/contracts.md: add screenshots.compression field
• docs/integration-guide.md: add compression example with typical savings
• CHANGELOG.md: add entry under Unreleased

7. GitHub Actions updates

• actions/baseline/action.yml: add optional install-pngquant input (default: true)
• Install pngquant in the action step when compression is enabled

Acceptance Criteria

• Config validation accepts screenshots.compression field
• Fast mode reduces file size by ~40-60% with no visually noticeable difference
• Aggressive mode reduces file size by ~70-80%
• Compression is optional (default: none, no change to existing behavior)
• Missing pngquant logs a warning but doesn't fail the capture
• Types updated
• Tests added and passing
• Docs updated
• npm run ci passes on the branch