Add KiCad PCBnew WASM build support and documentation

Build infrastructure:
- Update build-pcbnew.sh with stub library compilation
- Update build-wxuniversal-wasm.sh with library symlinks and stubs
- Add stub libraries for libgit2 and curl (network ops via JS)
- Add WASM-specific CMake Find modules

Documentation:
- Add KICAD_WASM_BUILD.md with complete modification list
- Document 68 KiCad files modified for WASM support
- Document disabled features and reasons
- Document stub libraries and wxWidgets changes

Submodule updates:
- kicad: Add WASM/Emscripten build support (wasm-port branch)
- wxwidgets: Add KiCad compatibility fixes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

wasm/cmake/Findngspice.cmake

@@ -0,0 +1,25 @@
+# Findngspice.cmake - Stub for WASM builds
+# ngspice is not available in WASM builds (KICAD_SPICE=OFF)
+# We provide stub values so CMake configuration succeeds
+
+if(EMSCRIPTEN OR NOT KICAD_SPICE)
+    message(STATUS "ngspice not available for WASM build (SPICE disabled)")
+
+    # Set variables to indicate ngspice is "found" but disabled
+    set(ngspice_FOUND TRUE)
+    set(NGSPICE_FOUND TRUE)
+
+    # Provide empty values
+    set(NGSPICE_INCLUDE_DIR "")
+    set(NGSPICE_LIBRARY "")
+    set(NGSPICE_LIBRARIES "")
+
+    mark_as_advanced(NGSPICE_INCLUDE_DIR NGSPICE_LIBRARY)
+else()
+    # For non-WASM builds, use the system findngspice
+    find_path(NGSPICE_INCLUDE_DIR ngspice/sharedspice.h)
+    find_library(NGSPICE_LIBRARY NAMES ngspice)
+
+    include(FindPackageHandleStandardArgs)
+    find_package_handle_standard_args(ngspice DEFAULT_MSG NGSPICE_LIBRARY NGSPICE_INCLUDE_DIR)
+endif()