Publish a machine-validatable model export schema

[Singularity-tian]

Problem

MODEL_FORMAT.md documents the versioned Point/Line/Face/Volume/Root snapshot, and tests cover current exports, but external integrations do not have a committed JSON Schema they can validate independently.

Scope

Generate and commit a JSON Schema for the redacted public model export contract, plus a synthetic golden export. Keep raw personal text out of both artifacts.

Acceptance criteria

• A committed schema validates the current redacted synthetic export.
• Schema versioning and compatibility rules match MODEL_FORMAT.md.
• CI fails when generated schema drifts from the code model.
• The golden fixture contains Point, evolution/relation Line, Face, Volume, Root, and receipts using synthetic values only.
• persome model export output validates outside the source checkout.
• Integration documentation shows a minimal validation command.

Non-goals

This issue does not define a benchmark dataset, hosted API, or remote sync protocol.