Define scene format (litmus-h4yq)

Define the scene format: annotated sample content with semantic color references.

Summary of Changes

Added scene module to litmus-model with:

• ThemeColor enum: semantic color references (Foreground, Background, Cursor, Selection*, Ansi(0-15)) with resolve() method
• TextStyle: bold/italic/underline/dim modifiers
• StyledSpan: text + optional fg/bg ThemeColor + TextStyle, with builder methods
• SceneLine: sequence of StyledSpans
• Scene: id, name, description, and lines — a complete terminal scene definition
• All types are serde-serializable for data-driven scene definitions