Add ROADMAP.md with revised Phase 1 architecture

[brendancol]

Summary

• Adds ROADMAP.md matching the updated Phase 1 architecture issues (Phase 1: Decompose InteractiveViewer via Composition #61–Phase 1: Main loop cleanup — reorder phases, separate concerns #64)
• Phase 1 was revised after evaluating Numba's extending API (StructRef, @jitclass, @overload) against rtxpy's actual constraints (OptiX hardware ray tracing, small entity count, Python API interop)

Key decisions

• Phase 1: Decompose InteractiveViewer via Composition #61: Composition over ECS — subsystem classes held by InteractiveViewer, not a World entity registry
• Phase 1: Geometry layer manager — flat grouping with ordered visibility #62: Flat geometry layer manager over scene graph — matches OptiX's flat IAS model, hardware BVH already handles frustum culling
• Phase 1: Declarative key-binding map with direct dispatch #63: Declarative key-binding dict over pub/sub event bus — one input source + one consumer doesn't need decoupling
• Phase 1: Main loop cleanup — reorder phases, separate concerns #64: Loop phase reordering over GameLoop class — fix the input lag bug and separate concerns, no fixed-timestep accumulator

Test plan

• Verify ROADMAP.md renders correctly on GitHub
• Confirm content matches updated issues Project roadmap: architecture, rendering, simulation, platform, digital twin #57, Phase 1: Decompose InteractiveViewer via Composition #61, Phase 1: Geometry layer manager — flat grouping with ordered visibility #62, Phase 1: Declarative key-binding map with direct dispatch #63, Phase 1: Main loop cleanup — reorder phases, separate concerns #64