fix(verify): arm64 stack save, adaptive canary and host gating

Assisted-by: GLM 5.3

verify/call_arm64.go

@@ -34,6 +34,9 @@ const stackPad = 64
 // Call invokes the assembled function at fnAddr with the given ABI0 argument
 // block.  On arm64, the ABI0 convention places arguments on the stack starting
 // at [SP+8] (same as amd64 ABI0 in this project).
+//
+// Not safe for concurrent use: only one JIT call may be in flight at a
+// time, the trampolines keep the saved registers in package globals.
 func Call(fnAddr uintptr, args []byte) ([]byte, error) {
 	stackSize := stackPad + 8 + len(args) + 64
 	stackMem, err := syscall.Mmap(-1, 0, stackSize,