Share typing representation and GenericAlias controls - #9711

Summary

Align GenericAlias controls and share the Python 3.15 type-representation writer across GenericAlias, Union and constant annotation evaluators. The combined change spans three Rust files (+163/-182).

• Release the iterator's mutex before constructing its starred alias or dropping the original owner, so subclass finalizers can reenter the exhausted iterator.
• Preserve raw tuple-subclass equality results and CPython's inequality handling. Reconstruct the actual GenericAlias subtype in reduce/copy/pickle paths.
• Replace three divergent type formatters with one WTF-8 writer. Preserve optional-attribute lookup errors and short-circuits, non-string names, surrogate characters, and temporary-owner cleanup. Attribute probes release their values before the next callback.
• Preserve the public writer's extra __str__ call on returned str subclasses, after name-attribute cleanup. This call can change output or raise and must not be replaced with raw string copying.
• Match annotation tuple formatting, including (int) for a singleton tuple, and preserve Unicode in ConstEvaluator's own repr and Union's subclassing diagnostic.

Validation

Pinned reference: CPython d220409a13b77de7a6c30abe16ea90db01528edc, including typevarobject.c, genericaliasobject.c and unicode_writer.c.

• Independent source/ownership review passed. The expanded independent oracle passed 36 groups, a separate focused oracle passed 18 groups, and the previous 10 GenericAlias groups still passed. These are external CPython reference checks, not native RustPython runtime results.
• Final threaded cargo check passed (1m07s) and all-targets Clippy passed (2m21s), with exactly the same 12 existing baseline diagnostics and no new warnings.
• Rustfmt, diff checks and normal commit hooks passed without bypass.

Limits and integration

Native RustPython execution remains unperformed because its required safety guard is not verified; hosted runtime CI remains necessary. The separate non-threaded/WASM graph and full workspace/C-API test suite were not run under the current disk constraint. No repository tests, markers, Lib files or workflows changed.

#9714 separately fixes Union construction and comparison; #9687 fixes TypeAliasType constructor binding; #9700 changes qualified-name diagnostics. Exact-source composition with those changes was clean and reviewed, but the combined branches were not compiled. Broader payload ownership work in #8944/#8939 may require textual reconciliation and does not implement these controls.