Match Qwen reasoning history across effort changes

docs/issue117-reasoning-history/unit.log

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+
+running 3 tests
+test engine::tokenizer::issue117::direct_history_omits_reasoning_but_retains_the_scaffold ... ok
+test engine::tokenizer::issue117::issue117_history_renders_tokens_and_context ... ignored, CPU-only; installed Qwen tokenizer and pinned history receipts
+test engine::tokenizer::issue117::only_affected_histories_change_keys ... ok
+
+test result: ok. 2 passed; 0 failed; 1 ignored; 0 measured; 557 filtered out; finished in 0.00s
+