[Gambit-Checkmate]

The change

Closes #660.

Treat None as the omitted iterable rather than truth-testing every input. BarrelList currently silently drops a false-valued iterable's contents and rejects a valid iterable whose truth value is ambiguous. Iteration itself now determines contents, as for builtin list. Existing explicit-None behavior remains empty; invalid false non-iterables such as 0 now raise TypeError.

Three regressions fail before the change: nonempty false-valued iterable, ambiguous-truth iterable, false non-iterable. Four controls cover None and empty list/tuple/iterator. This does not alter pending #617's copy/pickle reduction or #613's insertion normalization.

Validation on Windows / CPython 3.13.13:

• Full unit/doctest suite: 781 passed, 1 skipped, 22 subtests passed.
• New-test Ruff E9/F and format, compileall and git diff checks passed.
• Full listutils Ruff E9/F has three pre-existing F841 locals in setitem, as on pristine master; not altered here.
• Wheel and sdist builds passed. No version bump.
• Other platforms/interpreters await upstream CI; no coverage percentage claimed.

The backstory

Previous contribution #601 was acknowledged and incorporated in #609. This follow-up came from inspecting current master and comparing documented container behavior with builtin containers, not personal production usage. Existing related contributions were checked before filing the linked issue.

AI assistance

• Harness/tooling: OpenAI Codex, Git, GitHub CLI/API, Python, pytest and Ruff. Codex performed investigation, implementation, testing and submission.
• Model(s): GPT-6 family; the exact deployment identifier is not exposed in this session.
• Original prompt(s), relevant verbatim excerpts from the daily automation (the mailbox workflow and local memory path are omitted):

每天早上 8 点在本会话实际提交两个高质量的开源贡献 PR，把创建并获得 GitHub PR 链接作为每日完成目标。首批候选不合适时必须继续扩大仓库与问题搜索范围，不因候选重复、单个问题困难或尚未发现合适 issue 就结束；没有合适公开 issue 时可以检查代码、复现真实缺陷，确认无重复后报告问题并提交独立修复。

只有改动真实、有依据、验证充分且账号有权限时才推送分支并创建 PR；绝不降低 star 门槛，不提交空改动、无关改动、未经验证或重复 PR，尊重上游 AI 贡献和认领规则。

Hosted CI: Actions run 40424154552 is action_required for external-contribution approval. No hosted tests have executed for this head.