Add via-in-pad autorouter configuration

[AnasSarkiz]

Summary

• add optional allow_via_in_pad to pcb_group.autorouter_configuration
• add focused schema coverage for the new flag
• regenerate the Circuit JSON README types

Why

Via-in-pad routing can add fabrication cost, so downstream autorouters need an explicit opt-in that survives Circuit JSON serialization. Omitted or false remains disabled; only an explicit true will enable the downstream behavior.

This is the Circuit JSON companion to tscircuit/props#759.

Compatibility

The new field is optional and backward-compatible.

Validation

• bun test — 271 pass, 0 fail
• bunx tsc --noEmit
• bun run build
• bun run lint:zod
• bun run check-snake-case
• changed-file Biome formatting
• bun run generate-docs

[seveibar]

Not needed?