Add a native Container.__contains__

[tfoutrein]

Summary

First of the small, self-contained, conformance-preserving performance changes discussed in #483.

Container inherits __contains__ from MutableMapping, so key in container goes through self[key] → __getitem__ → item(), which resolves and returns the value — and constructs a NonExistentKey exception on every absent key — only to discard it.

This adds a native Container.__contains__ that resolves the key exactly as item() does (str → SingleKey; a non-str/non-Key argument still raises TypeError) and probes self._map directly. For an out-of-order table (a tuple index) it still builds the OutOfOrderTableProxy, so its validation runs exactly as before. This also speeds up Container.__setitem__, which does if key in self on every assignment.

Behaviour — no change

Beyond the existing suite, I ran a differential in check (present / absent / out-of-order / dotted-string / non-str keys) comparing this branch against master: identical results, including the TypeError on a non-string key and the out-of-order proxy path.

Benchmark

Drift-immune A/B in a single process (the new __contains__ vs the inherited __getitem__-based path, interleaved so thermal drift cancels), CPython 3.11, on a mix of present + absent keys:

old (inherited via __getitem__): ~610 ms
new (native __contains__):       ~520 ms
→ ~1.17× faster  (reproducible: 1.17× across runs)

The win is mostly from not constructing/raising NonExistentKey on absent-key membership.

Verification

• pytest tests/ (incl. the toml-test conformance submodule): 969 passed, unchanged from master.
• ruff check + ruff format --check: clean.
• mypy (1.19.1): no new errors. The single # type: ignore[arg-type] mirrors the existing SingleKey(key) call in item() (which also raises TypeError on a non-string argument).

---

_{Context: this is harness-assisted work — every change is individually benchmarked and gated (full test suite incl. conformance, a differential guard, ruff + mypy) before it can land, and I review and stand behind each PR personally. Happy to slice the rest of #483 into further small PRs.}