[Tristan-Stoltz-ERC]

Closes #2866.

What this starts

• Defines Symtropy as the deterministic physical-context and perturbation layer around a quantum workload, not as a quantum-state simulator.
• Documents a three-layer boundary between Symtropy geometry/dynamics, qualified physical adapters, and Symthaea QARCH-001/QARCH-002/QARCH-003 models.
• Adds an analytic point-to-point geometry fixture with a 1 mm detector displacement and independently derived expected Euclidean distance.
• Explicitly blocks inference from geometry alone to optical transmission, phase, coherence, detector performance, or quantum advantage.
• Defines validation tiers and negative controls, and coordinates with VIS-OPTICS-001 (VIS-OPTICS-001: add deterministic optical forward-model qualification fixtures #1807) rather than proposing a competing optical framework.

Evidence / validation status

• The JSON fixture was constructed and parsed in the authoring tool; the analytic value was independently calculated from sqrt(0.2^2 + 0.001^2) = 0.20000249998437522 m.
• Source and fixture paths were verified on this branch after creation.
• No Symtropy Rust code or tests were executed in this environment. This is a documentation/fixture draft, not a passing simulation qualification and not hardware validation.

Deliberate boundary

This PR does not add wave optics or quantum state evolution, does not add a heavy dependency, and does not claim the rigid-body engine can predict photon statistics. Geometry-to-optics coupling must be separately modeled and calibrated.