[Stack 2/4][Draft] Gemma4 core manual follow-ups (processor/comments)

[lesj0610]

internal stacked PR for branch organization only.\n\nthis layer contains manual follow-ups added during PR preparation, not a direct response to one upstream review thread.\n\nmain contents:\n- comment/assumption cleanup\n- processor tokenizer reassignment fix (44ea194)\n\nupstream-facing branch: lesj/gemma4-core-pr\nupstream PR: vllm-project#40281

[github-actions]

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[lesj0610]

superseded by the finer-grained Gemma4 core stack in #16-#22.