feat: Add SimulationSpeedGoldenTest — Phase 4.1 golden output tests for speed control semantics

Adds 7 integration tests in desktop-ui/src/test/.../gui/SimulationSpeedGoldenTest.kt validating:
- All speed multipliers (0.5x, 1x, 2x, 10x, 50x) produce identical simulation semantics
- Event sequences (timestamps + types) identical across all speed multipliers
- RealTimeSynch at 50x does not alter simulation semantics
- Physics invariants: velocity >= 0, acceleration in [-3.0, 4.0] m/s²
- Train front-positions within 1e-6 m tolerance across all speed multipliers
- Stochastic train generation (exp(43), seed=0) is reproducible across runs
- SimulationRunner speed multiplier does not alter simulation output

All 7 tests pass (total suite: 555 tests, 554 passing, 1 skipped).

Agent-Logs-Url: https://github.com/bedaHovorka/interlockSim/sessions/4cf5facd-49b9-4983-aa91-b60b661838d9

Co-authored-by: bedaHovorka <5263405+bedaHovorka@users.noreply.github.com>

Author: Copilot