Fix spurious KeyAlreadyPresent on valid out-of-order child table (#571)

[chiliec]

Fixes #571.

Problem

A regression in 0.15.1 rejects a valid document when a table has an out-of-order child, its concrete parent is declared afterwards, and another table separates a later sibling child:

import tomlkit

tomlkit.loads("""\
[tool.ruff]
[tool.ruff.lint.a]
[tool.ruff.lint]
[[tool.poetry.source]]
[tool.ruff.lint.b]
""")
# tomlkit.exceptions.ParseError: Key "lint" already exists. at line 5 col 0

tomllib and tomlkit 0.13.3 / 0.14.0 / 0.15.0 all accept it.

Root cause

The concrete+super validation added in 0.15.1 (_validate_table_candidate) treats only Table/AoT as table-like:

if isinstance(existing, (Table, AoT)) != isinstance(v, (Table, AoT)):
    raise KeyAlreadyPresent(k)

When lint is spread across out-of-order headers, current.value.item("lint") returns an OutOfOrderTableProxy, not a Table. So the left side is False while the candidate [tool.ruff.lint.b] is a Table (False != True), and valid input is reported as a type conflict.

Fix

• Recognise OutOfOrderTableProxy as table-like in the type-conflict check.
• When the existing entry is a proxy, recurse into each of its concrete Table fragments (existing._tables) so a genuine duplicate is still detected, rather than rejecting outright.

Real duplicates — including out-of-order ones like [a.b] / [a] / [a.b] — remain rejected.

Test

Adds test_parse_accepts_out_of_order_child_with_intervening_table (the issue's exact document; asserts the parsed structure matches tomllib and the round-trip is byte-for-byte preserved). Also adds a CHANGELOG entry.

Verification (local, Python 3.11)

• RED→GREEN: with the fix reverted the new test fails (Key "lint" already exists); with it, it passes.
• pytest tests/ — 1053 passed, 0 failed (full suite incl. the BurntSushi toml-test conformance corpus), no regressions.
• The parsed result equals tomllib.loads(...) and dumps(doc) reproduces the input exactly.
• ruff check introduces no new lint errors in the changed regions.

[dimbleby]

explain why not #572