fix(quad): throw on zero/invalid pitch instead of emitting NaN pad coordinates

[billythompsons]

Fixes #871

Problem

Passing an explicit zero pitch to a quad-family footprint (lcc_p0mm, qfn16_p0mm, etc.) silently produced NaN pad coordinates. In quadTransform, the underspecified-dimension fallbacks gate on truthiness of v.p, so p = 0 skips them: w/h stay undefined and pw/pl fall through to undefined * 0.6 = NaN. getQuadCoords then emits NaN x/y, which JSON.stringify turns into null - silently invalid circuit-json with no error.

Fix

Validate the effective pitches (p, px ?? p, py ?? p) at the end of quadTransform and throw a descriptive error when any is non-finite or <= 0:

Invalid p (0) for lcc: pitch must be a number greater than 0

A zero-pitch package has degenerate geometry (all pads on the same line), so a clear thrown error is the right outcome - this matches the error style already used in dfn.ts/lga.ts transforms.

Tests

New tests/quad-zero-pitch.test.ts:

• lcc_p0mm, qfn16_p0mm, qfn16_px0mm throw with the new message
• valid quad footprints (lcc, qfn16, qfn16_p0.4mm, lcc_p1mm) still produce finite pad coordinates

Full suite: 566 pass, 0 fail. biome format clean.