SimReady add-on install and authoring

Work on a caller-owned copy. The official SimReady add-on owns authoring, validators, and export semantics. The pinned OVRTX example consumes supported SimReady state but does not replace that add-on.

Preflight

1. Confirm the Blender/add-on versions supported by the supplied distribution.
2. Run scripts/probe_simready_surface.py inside the target Blender. It reports the installed module and exact registered operator candidates without mutating the scene. A missing required authoring surface is blocked.
3. Read references/simready-public-contract.md before scripting. Probe operator RNA in the running version; never infer an operator from an old screenshot or invent equivalent custom properties.
4. From the public/ distribution root, run: