fix: parse imperial/metric sizes for led and diode string footprints

[64johnlee]

Summary

Fixes #562 - String parser failed for led and diode with imperial sizes (e.g., led0402, diode0603).

Root Cause: In src/footprinter.ts, the proxy handler only recognized res and cap as passive footprints that use imperial/metric sizing. When parsing led0402, it was treated as { fn: "led", num_pins: 402 } instead of { fn: "led", imperial: "0402" }.

Fix: Extended the condition to also include diode and led:

// Before
if (prop === "res" || prop === "cap") {

// After  
if (prop === "res" || prop === "cap" || prop === "diode" || prop === "led") {

Test plan

• fp.string("led0402").json() now correctly returns { fn: "led", imperial: "0402" }
• fp.string("diode0603").json() now correctly returns { fn: "diode", imperial: "0603" }
• All 412 existing tests pass

Closes #562