Architecture: Prototype JSON theme loading with Borland3D as proof of concept

[sfloess]

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

Scope

Port the Borland3D theme to JSON format and build a minimal loader in curses-java to validate the approach end-to-end before committing to full migration.

Tasks

• Create borland3d.json theme file from the existing Java Borland3D implementation
• Build a JsonThemeLoader class in curses-java that reads a JSON theme file and produces a Theme3D instance
• Verify the JSON-loaded theme produces identical rendering output to the hardcoded Java implementation
• Document any data that cannot be expressed in JSON (e.g., custom rendering callbacks)
• Write tests comparing JSON-loaded vs hardcoded theme behavior

Acceptance Criteria

• Borland3D renders identically whether loaded from JSON or hardcoded
• Loader handles missing optional fields gracefully
• Performance acceptable (theme loading is a one-time cost, but verify no regression in rendering)
• Any gaps in the JSON schema identified and fed back to the schema sub-issue