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/stubs/curl_stub.c

@@ -0,0 +1,15 @@
+/* CURL stub for WebAssembly builds
+ * Network operations need to go through JavaScript fetch API
+ */
+
+typedef int CURLcode;
+#define CURLE_OK 0
+
+CURLcode curl_global_init(long flags) {
+    (void)flags;
+    return CURLE_OK;
+}
+
+void curl_global_cleanup(void) {
+    /* Nothing to clean up */
+}