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_stub.c

@@ -0,0 +1,66 @@
+/*
+ * NNG stub implementation for KiCad WASM build
+ * Socket-based IPC is not supported in browsers.
+ * These stubs allow the code to compile but return errors indicating unsupported.
+ */
+
+#include "nng/nng.h"
+
+// All NNG operations fail in WASM since sockets aren't available
+// Using NNG_ENOENT as a generic "not available" error
+
+int nng_rep0_open(nng_socket *s) {
+    if (s) {
+        s->id = -1;
+    }
+    return NNG_ENOENT;
+}
+
+int nng_listener_create(nng_listener *l, nng_socket s, const char *url) {
+    (void)s;
+    (void)url;
+    if (l) {
+        l->id = -1;
+    }
+    return NNG_ENOENT;
+}
+
+int nng_socket_set_ms(nng_socket s, const char *opt, nng_duration dur) {
+    (void)s;
+    (void)opt;
+    (void)dur;
+    return NNG_ENOENT;
+}
+
+int nng_listener_start(nng_listener l, int flags) {
+    (void)l;
+    (void)flags;
+    return NNG_ENOENT;
+}
+
+int nng_recv(nng_socket s, void *data, size_t *sizep, int flags) {
+    (void)s;
+    (void)data;
+    (void)sizep;
+    (void)flags;
+    return NNG_ENOENT;
+}
+
+int nng_send(nng_socket s, void *data, size_t size, int flags) {
+    (void)s;
+    (void)data;
+    (void)size;
+    (void)flags;
+    return NNG_ENOENT;
+}
+
+int nng_close(nng_socket s) {
+    (void)s;
+    return 0;  // Close always succeeds
+}
+
+void nng_free(void *ptr, size_t size) {
+    (void)ptr;
+    (void)size;
+    // Nothing to free in stub
+}