fix(lga): prevent corner pad overlap with derived body size

[w1ne]

Problem

For LGA footprints where the body size is derived (i.e. w/h are not given explicitly), corner pads overlap with 0mm clearance, tripping pad-to-pad DRC inside the footprint itself and blocking the autorouter.

Repro:

fp.string("lga14").circuitJson() // corner pads 1 & 14 touch (0.00mm clearance)
fp.string("lga8").circuitJson()  // corner pads 1 & 8 touch (0.00mm clearance)

Root cause

In src/fn/lga.ts the default body size was derived as:

const width = parameters.w ?? (grid.y - 1) * parameters.p + 2 * parameters.pl
const height = parameters.h ?? (grid.x - 1) * parameters.p + 2 * parameters.pl

The inner edge of the left/right pads sits at (width - 2*pl) / 2 = (grid.y - 1) * p / 2, which is exactly the center line of the outermost top/bottom pads (and vice versa for height). The corner pads on perpendicular sides therefore overlap by pw/2 in both axes — edge-to-edge clearance is 0.

Measured before (default p=0.5mm, pw=0.28mm, pl=0.7mm):

• lga14 (4x3 grid): pad 1 at (-0.85, 0.75) 0.7x0.28 vs pad 14 at (-0.5, 1.1) 0.28x0.7 → 0.00mm clearance
• lga8 (2x2 grid): pad 1 at (-0.6, 0.25) vs pad 8 at (-0.25, 0.6) → 0.00mm clearance

Fix

Add a corner allowance of pw + 2 * 0.2mm to the derived body size when pads exist on adjacent sides, so corner pads get 0.2mm clearance:

• lga14: derived body 2.40x2.90mm → 3.08x3.58mm; min pad-to-pad clearance 0.00mm → 0.22mm (corner clearance 0.20mm)
• lga8: derived body 1.90x1.90mm → 2.58x2.58mm; min pad-to-pad clearance 0.00mm → 0.22mm (corner clearance 0.20mm)

Explicitly provided w/h are untouched, and two-sided grids like lga6_grid3x0 (no adjacent-side pads) keep their previous derived size, so no existing snapshots change.

Tests

• New regression test lga14 and lga8 keep clearance between corner pads asserts min pad-to-pad clearance > 0.1mm for both default footprints, with SVG snapshots per repo convention.
• Full suite: 522 pass, 0 fail.