M1: Screenshot Data Model (litmus-b10b)

Add screenshot.rs to litmus-model with Provider, Fixture, ScreenshotKey, ScreenshotMeta, ScreenshotManifest types. Wire into lib.rs. Add serde round-trip tests.

Summary of Changes

Added crates/litmus-model/src/screenshot.rs with:

• Provider — terminal emulator metadata (slug, name, optional version)
• Fixture — reproducible terminal scenario (id, name, description)
• ScreenshotKey — composite key (provider, theme, fixture slugs), flattened in JSON
• ImageFormat — Png | Webp with extension/mime helpers
• ScreenshotMeta — full screenshot record with URL, dimensions, format, timestamp, SHA-256 checksum
• ScreenshotManifest — top-level index with find/for_provider/for_theme/build_index query methods

17 tests covering: serde round-trip, find, URL construction, cache busting, index building, JSON shape verification.

Wired into lib.rs as pub mod screenshot;.