Implement Demo Architecture v2: Probar-First

[noahgift]

Summary

Implement the demo architecture v2 spec from docs/specifications/demo-architecture-v2.md.

Root Cause (from Five-Whys Analysis)

TUI and WASM demos don't share exact same logic because:

1. No probar-first testing enforcement
2. YAML configs exist but demos don't load them
3. No unified DemoEngine trait
4. Separate TUI/WASM state wrappers

Tasks

• Create src/demos/engine.rs with DemoEngine trait
• Create tests/probar_demos.rs with parity test
• Create examples/experiments/orbit_earth_sun.yaml
• Implement OrbitalEngine: impl DemoEngine
• Implement TspEngine: impl DemoEngine
• Create generic renderers
• Remove hardcoded KeplerConfig::earth_sun() calls
• Verify probar parity tests pass

Acceptance Criteria

cargo test --test probar_demos

All tests must pass.

[noahgift]

Triage: P3. Feature request — DemoEngine trait for probar-first parity between TUI and WASM demos. Architectural improvement, no bug.

[noahgift]

Triage 2026-03-31: Feature request — leave open. No work in progress.