Add minimal routing constraints for differentiable optimization

[ThomasPluck]

Summary

• Adds model/constraints.json with DRC-derived routing rules (width, spacing, radius) for use with playdough's constraint optimization flow
• Minimal rule set (<=20 rules) covering routing/waveguide/metal layers only

Test plan

• Schema validated via playdough.constraints.RoutingConstraints.from_json()
• Integration test with playdough.Router(constraints=..., routing_layer=...)

🤖 Generated with Claude Code

Summary by Sourcery

New Features:

• Introduce a DRC-derived routing constraints file defining width, spacing, and radius rules for key routing, waveguide, and metal layers in sky130.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Introduces a new DRC-derived routing constraints JSON model for sky130, intended to be consumed by playdough’s differentiable routing/optimization flow, with a minimal rule set focused on routing, waveguide, and metal layers.

File-Level Changes

Change	Details	Files
Add minimal DRC-derived routing constraints JSON for sky130 consumable by playdough.	Create a new constraints.json file under the sky130/model directory to define routing constraints. Encode width, spacing, and bend radius rules for routing, waveguide, and metal layers in a schema compatible with playdough.constraints.RoutingConstraints.from_json(). Limit the constraint set to a small number of essential rules (≤20) to support initial differentiable routing experiments.	sky130/model/constraints.json

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