Route SRJ buses with path attraction

[seveibar]

Summary

• consume explicit SimpleRouteJson.buses
• physically order bus lanes from the more tightly clustered endpoint
• route each bus center-out
• attract each subsequent member to an already-routed adjacent path
• expose bus attraction radius and bonus options
• add an SRJ scheduling test and a dense BGA visual fixture

Why

The Rust router already contains direction-aware path attraction, but the WASM/SRJ orchestration never supplied bus membership or guide paths. As a result, tscircuit buses were routed as unrelated sequential nets.

BGA prototype

The visual fixture uses a 10×10 BGA at 0.8 mm pitch and routes four 6-bit buses from inner pads to four board-edge headers on four layers.

Five-run comparison after warm-up:

Metric	Baseline	Bus-aware
Routed traces	24/24	24/24
Autorouting errors	0	0
Vias	48	48
Bends	54	48
Routed length	345.934 mm	346.880 mm
Median solve time	1136.2 ms	1159.5 ms

The bus-aware result uses 11.1% fewer bends with the same via count, 0.27% more copper, and about 2% more median runtime.

Dependencies

• depends on Emit bus constraints in Simple Route JSON core#2883
• authoring API is introduced by Add bus routing constraint props props#758

The krt-wasm tests supply buses directly at the SRJ boundary so this PR can validate the consumer before the upstream releases land.

Validation

• cargo check --no-default-features --features wasm
• bun run build
• bun run typecheck
• bun test — 6 passing
• visual inspection of the 24-signal BGA snapshot
• git diff --check