Include emscripten/fiber.h at global scope before minicoro.h (#1256)

minicoro.h includes <emscripten/fiber.h> from inside its own
extern "C" block when building for Emscripten. That pulls Emscripten's
C++ headers (e.g. em_asm.h with its template specializations) in with C
linkage, and the build fails with "explicit specialization of undeclared
template struct '__em_asm_sig'".

Include <emscripten/fiber.h> in action_node.cpp, the only translation
unit that includes minicoro.h, before minicoro.h. The include guard then
prevents the second inclusion under extern "C". The vendored minicoro.h
is left untouched.

Fixes #1226

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: facontidavide

src/action_node.cpp

@@ -14,6 +14,11 @@
 #define MINICORO_IMPL
 #include "behaviortree_cpp/action_node.h"
 
+// minicoro.h includes this inside extern "C": include it first, at global scope.
+#ifdef __EMSCRIPTEN__
+#include <emscripten/fiber.h>
+#endif
+
 #include "minicoro.h"
 
 #include <iostream>