feat: implement SPDIP-28 footprint

[sungdark]

Summary

Implement SPDIP-28 (Shrink DIP, 28-pin) footprint for tscircuit/footprinter.

This adds a new spdip28 footprint that uses the existing dip function with fixed parameters appropriate for SPDIP-28 packages:

• 28 pins (14 per side)
• Row spacing: 7.62mm (0.3")
• Pin pitch: 2.54mm (0.1")

Usage

import { fp } from 'footprinter'

// Via method chain
const footprint = fp().spdip28().circuitJson()

// Via string
const footprint2 = fp().string('spdip28').circuitJson()

Bounty

• Issue: Implement SPDIP-28 #180
• Bounty: $10 USD
• Payment address: ETH 0x5a134107516ecccc83232711efa50eb4e87602c7

[graphite-app]

Critical bug: The exposed pad configuration accepts either true or { x: string, y: string } per the schema (lines 21-23), but the implementation only handles the boolean case and uses hardcoded dimensions. When users pass custom dimensions via object notation, they are ignored.

Fix: Handle both cases:

if (parameters.ep) {
  let epw: number
  let eph: number
  
  if (typeof parameters.ep === 'object') {
    epw = length.parse(parameters.ep.x)
    eph = length.parse(parameters.ep.y)
  } else {
    epw = 0.55 // default exposed pad width
    eph = 0.35 // default exposed pad height
  }
  
  pads.push(rectpad(["thermalpad"], 0, 0, epw, eph))
}

Suggested change

	if (parameters.ep) {
	const epw = 0.55 // exposed pad width
	const eph = 0.35 // exposed pad height
	pads.push(rectpad(["thermalpad"], 0, 0, epw, eph))
	}
	if (parameters.ep) {
	let epw
	let eph
	if (typeof parameters.ep === 'object') {
	epw = length.parse(parameters.ep.x)
	eph = length.parse(parameters.ep.y)
	} else {
	epw = 0.55 // default exposed pad width
	eph = 0.35 // default exposed pad height
	}
	pads.push(rectpad(["thermalpad"], 0, 0, epw, eph))
	}

Spotted by Graphite



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