fix(quad): validate pitch > 0 instead of producing NaN coordinates (#871)

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Summary

Fixes #871.

Passing an explicit zero or negative pitch to quad-family footprints (e.g. lcc_p0mm, qfn16_p0mm) resulted in v.p: 0 being treated as falsy, skipping dimension calculation and producing NaN pad coordinates that serialize as null in circuit-json without throwing.

Changes

• In src/fn/quad.ts, validate that p, px, and py must be strictly positive (> 0) in quadTransform and throw a descriptive error when <= 0 is received.
• Add test in tests/zero-pitch-quad.test.ts verifying that lcc_p0mm and qfn16_p0mm throw descriptive errors instead of emitting NaN coordinates.