Align partial construction and state controls - #9679

Summary

• Keep nested partial keyword dictionaries isolated and flatten only partials whose instance dictionary remains absent.
• Recognize native Placeholder values, validate trailing placeholders before flattening, and reuse shared constructor argument binding with lazy instance-dictionary allocation.
• Validate restored state before tuple-subclass callbacks or state changes, preserve supplied instance dictionary identity, and copy keyword subclasses without rehashing stored keys.
• Preserve absent versus allocated-empty dictionaries in reduction.

Invalid state is rejected before mutation. Successful field replacement remains sequential, matching CPython rather than claiming transactional reentrant updates.

Reference: CPython 3.15.0 partial construction and state restoration.

Validation

• Eight bounded CPython 3.15 reference groups passed, including fresh-dictionary, flattening, callback, identity, and invalid-state cases.
• Independent source review caught and resolved an eager-dictionary allocation issue; final formatting and whitespace checks passed.
• Final focused VM, host-env and stdlib check and strict Clippy with rustpython-stdlib/threading passed. Normal repository commit hooks passed.
• RustPython runtime, full workspace/C-API suites, and cross-platform execution were not run. No test, Lib, workflow, or marker edits.

AI assistance: OpenAI Codex performed source comparison, implementation, reference probes, and independent source review; model version was not exposed.