refactor(rlix-validation): read model_update_transport once

[TianyeGGBond]

Context

assert_rlix_topology reads args.model_update_transport in two places:

• C11 (inside if is_rlix_mode():) to enforce cpu_serialize in RLix mode
• S2 to decide has_gpu_staging (whether a GPU-staging transport is in play)

# C11
if is_rlix_mode():
    transport = getattr(args, "model_update_transport", "cuda_ipc")
    ...
# S2
transport = getattr(args, "model_update_transport", "cuda_ipc")
has_gpu_staging = _topology_has_non_colocate_engines(args) or transport == "cuda_ipc"

The C11 copy is scoped inside the is_rlix_mode() branch, so S2 can't reuse it and re-reads the same arg with the same default.

Change

Fetch transport once in the top-of-function derived block (alongside train, infer, engine_count, per_engine) and drop both local re-reads. Single source, no behavioral change.

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