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 DRC-derived routing constraints JSON covering routing, waveguide, and metal layers for use by the router.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Introduces a new minimal DRC-derived routing constraints JSON model for use by playdough’s differentiable routing/optimization flow, defining a small set of width, spacing, and bend-radius rules for routing, waveguide, and metal layers.

File-Level Changes

Change	Details	Files
Add initial DRC-derived routing constraints model consumable by playdough	Introduce a new constraints.json file under the model directory to define routing constraints Encode width, spacing, and bend-radius rules for routing, waveguide, and metal layers in a format compatible with playdough.constraints.RoutingConstraints.from_json Limit the constraint set to a small number of rules (<=20) to serve as a minimal viable constraint deck for the router	cspdk/model/constraints.json

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