Core: Coerce integer width when assigning to a struct field

handle_struct_field_assignment stored the evaluated value as-is, and since
expressions evaluate in i64 that meant any assignment to a sub-64-bit field
(dat.hits = 1 with hits: c_uint32) died in llvmlite with "cannot store i64 to
i32*". Apply the same implicit widening/truncation the local-assignment path
already has. Surfaced by the augmented-assignment struct-field test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L1PX8EuP9C3o3veWGA84RF

Author: r41k0u

pythonbpf/assign_pass.py

@@ -54,6 +54,14 @@ def handle_struct_field_assignment(
         logger.info(f"Copied string to char array {var_name}.{field_name}")
         return
 
+    # Same implicit widening/truncation as assignment to a local: expressions
+    # evaluate in i64, but a field may be narrower.
+    if isinstance(val_type, ir.IntType) and isinstance(field_type, ir.IntType):
+        if val_type.width < field_type.width:
+            val = builder.sext(val, field_type)
+        elif val_type.width > field_type.width:
+            val = builder.trunc(val, field_type)
+
     # Regular assignment
     builder.store(val, field_ptr)
     logger.info(f"Assigned to struct field {var_name}.{field_name}")