feat: implement PDIP-8 footprint via pdip/spdip aliases

[Guciolek]

Summary

Closes #371

Adds pdip and spdip as aliases for the existing dip footprint implementation, so that strings like pdip8, PDIP8, spdip16, pdip8_p1.27mm all resolve to the same underlying DIP footprint.

Changes

• src/footprinter.ts: Add two regex aliases in normalizeDefinition:
  • ^pdip(?=[\d_]|$) → dip
  • ^spdip(?=[\d_]|$) → dip
• tests/dip.test.ts: Add 4 test cases covering:
  • Lowercase pdip8 resolves to dip8
  • Lowercase spdip16 resolves to dip16
  • Uppercase PDIP8 resolves to dip8 (case-insensitive)
  • Parameterized pdip8_p1.27mm resolves to dip8_p1.27mm

Why aliases (not a new function)

PDIP (Plastic DIP) and SPDIP (Shrink Plastic DIP) are package name variants of the standard DIP family:

• PDIP: Same through-hole pad layout and 2.54mm row pitch as standard DIP — only the body material differs (plastic vs ceramic).
• SPDIP: Same row pitch as DIP (2.54mm / 0.1in) — slightly narrower body width but pin row spacing is preserved.

For PCB footprint purposes (KiCad JLCPCB libraries, IPC-2221 conventions), they are identical to standard DIP and re-using the existing implementation:

• Keeps the code surface minimal (no duplication of pad geometry)
• Avoids drift between DIP/PDIP/SPDIP implementations
• Matches how KiCad/Altium/Eagle typically treat these as variants of the same footprint

Verification

All 416 existing tests in the footprinter repo pass:

$ bun test
 416 pass
 0 fail
 3 snapshots, 1040 expect() calls

Plus 4 new test cases for the aliases.

/claim #371