fix: reject disconnected via route transitions

[seveibar]

Summary

• require every pcb_trace via route point to be immediately bracketed by wire points
• keep the existing coordinate-alignment check and apply it to every valid via transition
• verify that adjacent wires lie within the via's inclusive physical layer span, including inner-layer taps on full-stack through vias

Why

checkTracesAreContiguous previously inspected a via only when both neighboring route points were already wires. A malformed route such as wire -> via -> via -> wire therefore skipped validation entirely, even though no copper segment connects the two vias. The same blind spot allowed edge vias and layer-mismatched transitions through the shared routing checks.

This was reproduced while auditing the manufacturing output in tscircuit/quick-configure: an initially prepared USB-C fan-in path contained consecutive vias and would have exported without the intended bottom-layer copper.

The layer check treats from_layer / to_layer as the physical plated span. For example, a top-to-bottom via may legitimately connect adjacent top and inner1 wire points.

Validation

• bun test — 184 pass, 0 fail
• bunx tsc --noEmit
• bun run build
• focused regression coverage for valid reversed spans, inner-layer taps on through vias, consecutive vias, and wires outside a via's physical span