reject a table redefined after its parent table header

[netliomax25-code]

Summary

1. Container.append sends a concrete [table] header that lands inside an existing implicit/super table through _validate_table_candidate, which only flagged a child key on a type change or a dotted-key redefinition.
2. A child table header that was already defined fell through to a plain merge, so a subtable redefined after its parent received its own header ([a.b] / [a] / [a.b], and the deeper [a.b.c] / [a] / [a.b.c]) was merged silently instead of rejected, while tomllib and the toml-test suite treat these as declaring the same table twice.
3. _validate_table_candidate now raises when the existing child and the candidate child are both concrete (non-super) tables, and recurses through matching super tables so a duplicate nested deeper is caught as well. Valid out-of-order tables (a new sibling subtable, or extending a table with a deeper header) keep parsing, and the existing tests plus the toml-test 1.0/1.1 suites pass.

Agent Drafting Metadata

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