Align object reduction and state callback controls - #9716

Summary

Align object reduction and state callback controls with Python 3.15 in one production Rust file (+186/-128).

• Bind the reduction protocol through existing index conversion to a signed C int, preserving the original temporary argument through reducer dispatch and using CPython's range error.
• Always perform dynamic __getstate__ lookup. Use the native default fast path only for the original implementation bound to the same receiver; retain instance overrides, foreign-bound methods, lookup errors and callback ownership.
• Preserve original slot-name objects as state dictionary keys. Release temporary names and values before rechecking slot-list size, and avoid unchecked indexing if a callback/finalizer changed that list.
• Propagate errors from newargs descriptors through the existing strict special-method lookup. Release returned outer tuples before validating their children, and preserve explicit argument/keyword cleanup on failures.
• Capture the current class at the correct point after callbacks, helper lookup and empty-keyword destruction. Release class-level reducer temporaries before invoking the instance reducer, and keep top-level result/failure cleanup in source order.

Existing binding, lookup, dictionary and method primitives are reused. No per-function keyword parser, test, Lib or workflow edits are introduced.

Validation

Compared with pinned CPython object reduction/state. The 29-case reference oracle reproduced exactly. Independent source review and additional safe failure, copyreg, class-mutation, slot-finalizer and temporary-protocol probes passed. Ordinary classes/subclasses and legal class reassignment were used; no malformed native layouts or unsafe cycles were exercised.

These are CPython reference results and source review, not a RustPython runtime pass. Native RustPython execution, full workspace/C-API suites and separate cross-platform graphs remain unperformed; hosted CI is required.

Limits and integration

Generic nested tuple member destruction still differs from CPython's reverse order. This patch fixes the changed top-level owners but does not redesign generic tuple teardown or claim full nested-container finalizer parity.

Fresh relevant PR patches were checked and do not duplicate this group. Composing with #9715 requires resolving the object.rs import hunk by retaining both its PyAnyDict import and this patch's native-function/method-descriptor imports. The directory and reduction behavior hunks are separate; combined execution is not claimed. Preserve the independent ownership/type-name work in #8939/#8944/#9700 and nearby #8848 length-access changes.

AI-assisted implementation and independent review using OpenAI Codex; model version is not exposed.

Final validation: threaded host-env/VM/stdlib cargo check passed in 1m12s and all-targets Clippy in 3m03s. Diagnostics exactly match the 12 unchanged baseline warnings. Normal repository commit hooks passed; no suppressions or unrelated baseline edits.