feat: add via-to-pad clearance DRC check

[Timmmmmo]

Summary

Implements check-via-pad-clearance that detects when a via is placed too close to a pad (SMT pad or plated hole) on a different net. Closes #44.

Key design decisions

• Comprehensive pad shape support: Uses getBoundsOfPcbElements so that all pad shapes (circle, rect, pill, rotated_pill, polygon, etc.) are handled correctly - not just circle and rect
• Via modeled as circle, pad as circle or bounding-box polygon: Leverages existing distanceBetweenCircleAndCircle / distanceBetweenCircleAndPolygon from circuit-json-util
• SpatialObjectIndex for efficient lookup: Same pattern as existing clearance checks
• Connectivity-aware: Vias connected to the same net as the pad are not flagged
• Exports getCircleShape and getPolygonShape from common.ts: Reuses existing helpers instead of duplicating code
• Local PcbViaPadClearanceError interface: circuit-json doesn't have this type yet; promoting it upstream would be a natural follow-up
• Registered in runAllRoutingChecks and index.ts

Test coverage (7 tests)

Test	Description
via too close to circle pad	Detects violation for circular pad
via far from pad	No false positive at distance
via too close to rect pad	Detects violation for rectangular pad
via too close to plated hole	Detects violation for plated hole
custom minClearance	Respects override clearance value
no vias	Returns empty for no vias
no pads	Returns empty for no pads

All 118 tests pass (7 new + 111 existing).