[P4] Geometry templates

[duckyquang]

Phase: P4
Owner(s): Davin

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Goal: Extend generator beyond equation systems to coordinate geometry.

Where: marc/data/templates.py — new CoordinateGeometry template family.

Parallel track: Independent of parser; uses existing schema + CAS.

1. Add geometry template: points as variables, distance/slope as factors.

2. Generate 20 samples; verify CAS accepts at x*.

3. Document in marc/data/README.md.

Done when: Geometry samples generate and pass CAS check.

Push to: PR davin/p4-geometry → main.

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