Architecture: Define canonical JSON schema for theme specifications

[sfloess]

Part of #233: Architecture: Unify curses-java and curses-themes with shared implementation

Scope

Design and define a JSON Schema that captures all theme data needed by both curses-java and curses-themes. This schema becomes the single source of truth for theme definitions.

Tasks

• Define JSON Schema covering: theme name/metadata, border characters (flat and 3D), color pairs (foreground/background for all component states), shadow/highlight/lowlight colors, shadow offsets, 3D rendering parameters
• Include support for both Theme (flat) and Theme3D (3D borders/shadows) variants
• Add validation rules (required fields, allowed color values, character constraints)
• Write 2-3 example theme JSON files (e.g., Borland3D, DOS, TRS-80)
• Validate examples against schema using a JSON Schema validator

Deliverable

• theme-schema.json - The JSON Schema definition
• 2-3 example .json theme files validated against it

Acceptance Criteria

• Schema covers all properties used by both Java and Python implementations
• Example themes validate against schema
• Schema is versioned (includes a $schema or version field)
• Human-readable and extensible for future theme properties