chore: 🔧 fork-cleanup tooling + bump kicad submodule

- kicad → a2c634f2: drop redundant wasm guards, restore upstream
  (modified upstream files 95→86, churn 3180→3104)
- kicad-diff-stats.sh: fix fork-point detection (add missing commit authors
  + an @emergence-engineering.com domain net) so it reports the real
  divergence instead of 0
- build-kicad-target.sh: drop dead -DKICAD_PCM and -DBUILD_GITHUB_PLUGIN
  flags (not options in this KiCad version); keep -DKICAD_SPICE=OFF, which is
  read by wasm/cmake/Findngspice.cmake

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/kicad/build-kicad-target.sh

@@ -451,9 +451,6 @@ emcmake cmake "${KICAD_DIR}" \
     -DODBC_LINK_FLAGS:STRING="" \
     -DODBC_LIBRARIES:STRING="" \
     \
-    -DBUILD_GITHUB_PLUGIN=OFF \
-    -DKICAD_PCM=OFF \
-    \
     -DHAVE_STRCASECMP=1 \
     -DHAVE_STRNCASECMP=1