Use endpoint pairs as the only via span API

[seveibar]

Use from_layer and to_layer as the sole via span API. Remove the optional via layers field and its precedence path; DRC and Circuit JSON conversion derive the inclusive board-layer span from required endpoints. Reversed endpoints produce the same geometry.

Replace explicit-layer tests with endpoint reversal and span-extension coverage, retaining intermediate-layer collisions, excluded layers, same-layer spans, and invalid endpoints. This is stacked on #97; merge that PR first.

Validation: all 89 tests pass (594 assertions) and typecheck passes. The autorouter consumer is updated in tscircuit/tscircuit-autorouter#2402.

[tscircuitbot]

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• /benchmark --scenario-limit 20 --max-iterations 100 -> run 20 samples with a max iteration override

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[tscircuitbot]

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