Keep TOML array storage synchronized during in-place repetition

[xiehuanyi]

Summary

In-place list repetition bypasses the storage used to render Array and AoT values. For example, a *= 2 changes an array's list contents, but doc.as_string() still writes the original elements; a *= 0 also leaves the old values in the serialized document. Arrays of tables similarly retain their original body.

Implement in-place repetition through the existing mutable-sequence operations so the list, rendered items, and table body stay synchronized. Preserve the object identity, support integer-index objects, and clear values for nonpositive factors. Add round-trip regressions for single-line/multiline arrays, arrays of tables, and subsequent edits.

Validation on Linux/Python 3.12: 12 cases fail before the fix; all 1,116 tests pass after the fix, including the pinned TOML compliance corpus. Repository-pinned Ruff 0.16.10 lint/format and git diff --check pass. Mypy 1.19.1 passes for the changed files. The optional whole-project mypy run reports the same five existing errors on an isolated unmodified checkout; it is not a repository CI step.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: GPT-6.1-sol, xhigh
• Notes: The reproduction, regressions, complete test suite, formatting/lint, and baseline comparison were run locally. Other platforms and the downstream Poetry integration matrix were not run locally.

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