feat: add single-layer and multilayer fanout autorouters

[seveibar]

Summary

• add autorouter="single_layer_fanout" and autorouter="fanout"
• register both modes through the local-autorouter strategy extension point now on main
• run @tscircuit/fanout-solver as a breakout-only stage that returns escape traces plus a transformed SimpleRouteJson
• pass that transformed problem to a separate default-autorouter follow-up stage
• read per-bus fanout policy from <autoroutingphase busFanoutDirections={...}>
• infer the source footprint without requiring a component selector
• preserve circular pad geometry for interstitial and thermal-pad escapes
• replace escaped source pads with fixed source-footprint keepouts for downstream routing
• keep <bus> limited to electrical connection membership
• bundle the fanout solver into core

Integration cleanup

• rebased onto main after feat: default breakout autorouter to fanout props#761 and refactor: extract local autorouter strategies #2898 merged
• reduced the PR to one fanout feature commit
• updated to @tscircuit/props@0.0.595
• made Breakout parse its runtime props with breakoutProps, so the schema-owned autorouter="fanout" default is honored without duplicating it in the constructor
• kept concrete autorouter selection in the strategy registry and simplified the multi-stage SimpleRouteJson handoff in Group

Breakout coverage

• two-layer <breakout> regression with a 6×6, 0.8 mm-pitch BGA surrounded on four sides by four 0603 decoupling capacitors
• four two-signal buses
• fanout phase produces eight boundary escape traces and eight vias
• downstream phase completes all 16 trace segments to the capacitors
• no PCB autorouting, trace-clearance, pad-clearance, or via-clearance errors under 0.1 mm trace/clearance rules and 0.2/0.5 mm via geometry
• full PCB and multi-stage SimpleRouteJson snapshots

Validation

• bunx tsc --noEmit
• focused fanout, breakout, bus, and preset tests
• full local suite: 1,276 passing, 31 skipped, 0 failing
• git diff --check
• PCB and phase-stack snapshots visually verified: fanout stops at the shared breakout boundary, then the default stage completes routing to the surrounding capacitors

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Project	Deployment	Actions	Updated (UTC)
tscircuit-core-benchmarks	Ready	Preview, Comment	Jul 28, 2026 6:22pm

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