fix: default num_pins to 2 for bare solderjumper

[Yingtm]

Summary

Fixes #784

Calling fp.string("solderjumper") without a pin count left num_pins as undefined, causing every value derived from (num_pins - 1) * padSpacing to be NaN. The courtyard rect and silkscreen text silently contained NaN/null coordinates.

Fix

Made num_pins optional in the function signature and default it to 2 via destructuring, matching the behavior of solderjumper2 and most other footprints that accept a pin count.

// Before:
const { num_pins, bridged, ... } = params  // num_pins = undefined → NaN

// After:
const { num_pins = 2, bridged, ... } = params  // num_pins = 2 → valid geometry

Testing

• Added test: bare solderjumper defaults to 2 pins
• All 6 solderjumper tests pass (0 failures)
• All 568 existing tests still pass

bun test tests/solderjumper.test.ts
# 6 pass, 0 fail