Add UFQFPN-20 footprint

[Abse2001]

Summary

• add an explicit ufqfpn20 footprint variant
• preserve its eight chamfered corner leads as polygon copper
• add a C35556 regression test and SVG snapshot

Motivation

STM8S003F3U6TR (JLCPCB C35556) uses a UFQFPN-20 land pattern with twelve rectangular leads and eight chamfered corner leads. Symmetric LGA/QFN pads cannot reproduce the corner copper, limiting the best match to 85.75% IoU.

Implementation

The package-specific UFQFPN-20 variant encodes the verified standard land pattern and its polygon corners. It exposes no speculative polygon or per-pad controls.

Validation

• bun test tests/ufqfpn.test.ts: 1 passed
• bun run build
• C35556 copper IoU: 100.00%
• new SVG snapshot showing all eight chamfered corner leads

[seveibar]

idgi, footprint doesnt support pin counts, is this an ipc standard