fix: throw clear error for zero/negative pitch in quad footprints

[Yingtm]

Summary

Fixes #871

Passing an explicit zero pitch (e.g. lcc_p0mm, qfn16_p0mm) to quad-family footprints silently produced NaN pad x/y coordinates, which JSON.stringify serialized as null — silently corrupting the circuit-json output.

Root Cause

In src/fn/quad.ts, quadTransform uses falsy checks like if (!v.p) to decide whether a value was explicitly provided. When p = 0 (a valid number but falsy), these checks incorrectly treat it as "not set," skipping:

• width/height derivation (v.w, v.h stay undefined)
• pad width/length derivation (v.pw, v.pl stay undefined)

This leaves getQuadCoords computing x: -w / 2 - pcdfe + 0.1 with w = undefined, producing NaN.

Fix

Added validation at the top of quadTransform that checks whether p, px, and py are positive numbers. If any is zero or negative, a clear error is thrown:

Error: Invalid pitch (p=0): pitch must be a positive number. Use a non-zero value like p0.5mm.

This prevents silent NaN propagation and gives the user actionable feedback.

Testing

• Added tests/quad-zero-pitch.test.ts with 4 test cases covering the fix
• All 568 existing tests still pass (0 failures)
• Reproduced the original bug before the fix, confirmed the error is thrown after

bun test tests/quad-zero-pitch.test.ts
# 4 pass, 0 fail

bun test
# 568 pass, 0 fail