[doubling] get

[ia7ck]

• max_steps = 0 で使うことはなさそう
• get()

[Copilot]

Pull request overview

libs/doubling の Doubling に、内部のダブリングテーブル（2^k 遷移）へ直接アクセスする get() を追加し、max_steps == 0 の取り扱いを明確化（現在は拒否）するPRです。

Changes:

• Doubling::new() で max_steps > 0 を assert! で強制
• Doubling::get(start, k) を追加して 2^k 遷移を参照で返却
• プロパティテストの入力範囲を max_steps >= 1 に変更し、新しい前提条件に合わせる

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[Copilot]

Doubling::new previously supported max_steps == 0 (and fold(start, 0, ...) still works), but this change introduces an unconditional panic for that input. If this crate is consumed externally, this is an API-breaking behavior change; consider keeping the old handling (treat 0 as a valid value) or documenting the new precondition clearly (e.g., in rustdoc / changelog) and making tests reflect the intended contract.

Suggested change

	assert!(max_steps > 0);
	let log2_max_steps = max_steps.ilog2() as usize;
	let log2_max_steps = if max_steps == 0 {
	0
	} else {
	max_steps.ilog2() as usize
	};