Enable 3D viewer in KiCad WASM build

Build the 3D viewer code for WASM instead of stubbing it out.
This is simpler to maintain than complex header stubs.

Changes:
- Enable KICAD_BUILD_3D_VIEWER_WASM=ON in build script
- Remove -gsource-map to avoid OOM in wasm-emscripten-finalize
- Add OpenGL stubs for fixed-function pipeline (WebGL ES 2.0 only)
- Add GLU stub for gluErrorString
- Add NNG stub for IPC API (sockets don't work in WASM)
- Add PCBnew scripting stub
- Expand curl and libgit2 stubs

The 3D viewer code compiles but won't render at runtime since
OpenGL GAL is disabled and Cairo GAL is used instead.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wasm/stubs/nng/protocol/reqrep0/rep.h

@@ -0,0 +1,12 @@
+/*
+ * NNG REP protocol stub header for KiCad WASM build
+ */
+
+#ifndef NNG_PROTOCOL_REQREP0_REP_H
+#define NNG_PROTOCOL_REQREP0_REP_H
+
+#include "../../nng.h"
+
+// nng_rep0_open is already declared in nng.h
+
+#endif // NNG_PROTOCOL_REQREP0_REP_H