feat: add via-to-pad clearance check

[harrrshall]

Closes #44

Adds a new DRC check, checkViaPadClearance, that flags vias placed too close to pads (pcb_smtpad and pcb_plated_hole).

Approach

• Treats each via as a circle (outer_diameter) and measures the edge-to-edge gap to every pad.
• Circular pads use circle-to-circle distance; rectangular pads use circle-to-bounding-box distance (consistent with how check-pad-pad-clearance treats rectangular pads).
• Skips via/pad pairs that share no copper layer.
• Skips vias connected to the pad's net (intentional proximity), using the connectivity map — matching check-via-trace-clearance.
• Default minClearance is the board's min_pad_edge_to_pad_edge_clearance DRC value, falling back to the JLC minimum; overridable via the options argument.

Registered in runAllRoutingChecks and exported from index.ts.

Tests

tests/lib/check-via-pad-clearance.test.ts covers a violation, a via far from the pad, a via connected to the pad's net, and a custom minClearance. All four pass locally (bun test).

Note

circuit-json does not define a pcb_via_pad_clearance_error type yet, so the error interface is declared locally in the check. Promoting it to circuit-json would be a sensible follow-up — happy to do that if preferred.