docs: add geometry requirements guide for optical photon simulation

[plexoos]

No description provided.

[Copilot]

Pull request overview

Adds a dedicated guide describing geometry authoring requirements and best practices for reliable optical photon simulation in this codebase, and links it from the top-level README to make it discoverable.

Changes:

• Add docs/geometry-requirements.md covering naming, hierarchy/factorization constraints, selective triangulation guidance, and validation expectations.
• Link the new guidance from README.md under a “Geometry Guidance” section.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/geometry-requirements.md	New guide documenting geometry constraints (analytic vs triangulated, instancing/factorization limits) and validation workflow for optical studies.
README.md	Adds a pointer to the new geometry guidance document for users authoring GDML/Geant4 geometry.

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[ggalgoczi]

1. Two simphony references at lines 4 and 257, Copilot flagged on 2026-04-23, still unresolved. Should be eic-opticks/simg4ox until/unless PR #298 lands.
2. Coordinate with the readthedocs branch, it already has docs/geometry.md and a Sphinx toctree. Either rename this PR's file (e.g. docs/geometry-authoring.md) so the layering is explicit, or replace the existing geometry.md. Either way, add it to docs/index.rst::toctree or it won't render on the docs site.
3. Add the explicit "required optical properties" list (RINDEX / GROUPVEL / EFFICIENCY / REFLECTIVITY + the WLS triplet) — currently requirement #7 vaguely says "required optical properties" without naming them.
4. Per-rule "how violations surface" notes, makes the doc actionable. E.g. rule #1 → silent mis-triangulation; rule #6 → photon leakage observable in GPUPhotonSource GPU/G4 comparison.
5. 2–3 small GDML snippets (clean repeated-LV subtree, isolated triangulation candidate, clean optical-boundary stack). All-prose makes the rules harder to apply. 6. Verify U4Mesh__NumberOfRotationSteps_* is the current API name (line 169-170).

[ggalgoczi]

My main concern with this PR is that it is misleading potential users. It claims triangulization is usable under current code. However AFAIK we never used it. Please either prove that it works or mention that is untested.