Core: Give a helper call's value the sign its registry entry declares

The registry types ktime, pid, uid, random, cgroup_id and smp_processor_id
as unsigned, and undeclared locals initialised from them already took
that sign. The expression value did not: each emitter returns
(value, plain i64), so ktime() >> 1 was an ashr and pid() // 3 an sdiv.
The dispatcher now substitutes the registered descriptor when the widths
agree, in one place instead of in every emitter.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: r41k0u

pythonbpf/helper/bpf_helper_handler.py

@@ -1076,14 +1076,27 @@ def invoke_helper(method_name, map_ptr=None):
             raise NotImplementedError(
                 f"Helper function '{method_name}' is not implemented."
             )
-        return handler(
+        result = handler(
             call,
             map_ptr,
             compilation_context,
             builder,
             func,
             local_sym_tab,
         )
+        # Emitters return (value, plain LLVM type); the registry entry is the
+        # descriptor that knows the sign. Substitute it once, here, rather
+        # than in every emitter.
+        declared = HelperHandlerRegistry.get_return_type(method_name)
+        if (
+            isinstance(result, tuple)
+            and len(result) == 2
+            and isinstance(declared, ir.IntType)
+            and isinstance(result[1], ir.IntType)
+            and declared.width == result[1].width
+        ):
+            return result[0], declared
+        return result
 
     map_sym_tab = compilation_context.map_sym_tab