[P3] Structure diffusion prototype

[duckyquang]

Phase: P3
Owner(s): Davin

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Goal: Prototype denoising graph structure (add auxiliary nodes). Preliminary only.

Where: marc/structure/diffusion.py, schema.py, scripts/train_structure_pilot.py.

Parallel track: Akash builds toy problems, Quang extends denoiser head, Sparsh writes H2 metrics — all Day 1 of P3.

1. Padded slot schema (ABSENT type) in structure/schema.py.

2. Categorical forward corruption in structure/diffusion.py (simple flip noise OK).

3. Pilot training script — 1000 steps max on toys.

4. Report: results/p3_structure/pilot_report.md.

Done when: One toy run shows a slot going ABSENT → active.

Push to: PR davin/p3-structure → main.

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