fix: acquire GIL before C→Go argument conversion in generated CGo wrappers

C.GoString (and other py2go converters) call runtime.gostring → mallocgc
inside a CGo callback. If those conversions run after PyEval_SaveThread
releases the GIL, Go's GC can observe a corrupted sweep-generation counter,
causing "fatal error: bad sweepgen in refill" on Go ≥1.24 / macOS x86_64
(issue #370).

In genFuncBody(), pre-convert each py2go argument into a local variable
while the GIL is held via PyGILState_Ensure/Release, then release the GIL
for the actual Go function call as before. Interface-handle arguments
(ifchandle && goname == "interface{}") are excluded from pre-conversion,
matching the existing callArgs switch logic to avoid type mismatches in
generated code for the iface example.

Also documents Idea 2 (unsafe.String zero-alloc approach) as a future
defence-in-depth option in a code comment.

Author: b-long

bind/gen_func.go

@@ -287,15 +287,15 @@ func (g *pyGen) genFuncBody(sym *symbol, fsym *Func) {
 	preConvertedArgs := map[int]string{} // arg index → temp variable name
 	needsGILForArgs := false
 	for _, arg := range args {
-		if arg.sym.py2go != "" && !arg.sym.isSignature() {
+		if arg.sym.py2go != "" && !arg.sym.isSignature() && !(ifchandle && arg.sym.goname == "interface{}") {
 			needsGILForArgs = true
 			break
 		}
 	}
 	if needsGILForArgs {
 		g.gofile.Printf("_gstate := C.PyGILState_Ensure()\n")
 		for i, arg := range args {
-			if arg.sym.py2go != "" && !arg.sym.isSignature() {
+			if arg.sym.py2go != "" && !arg.sym.isSignature() && !(ifchandle && arg.sym.goname == "interface{}") {
 				anm := pySafeArg(arg.Name(), i)
 				tmpVar := fmt.Sprintf("_arg%d", i)
 				g.gofile.Printf("%s := %s(%s)%s\n", tmpVar, arg.sym.py2go, anm, arg.sym.py2goParenEx)