Align Union construction and callback controls - #9714

Summary

• Build Union arguments through one streaming builder, preserving checked typing construction separately from unchecked type/GenericAlias operators. Subscription uses exact-tuple unpacking; substitution validates directly without invoking user __or__ or __ror__.
• Preserve creation-time hashable/unhashable partitions for equality and hashing. Freeze stored set entries without repeating hash or collision callbacks, and preserve identity shortcuts and containment comparison direction.
• Preserve callback errors and ownership: only the initial hash failure classifies an argument as unhashable; subsequent hash failures use the shared contextual formatter while comparison exceptions remain unchanged. Keep original inputs alive and release failed-builder lists in reverse order.

Validation

• Independent source/ownership review and 19 pinned CPython 3.15 oracle groups passed, covering callback order, exception identity, temporary lifetimes, partition equality and checked-entry-point differences.
• Final threaded cargo check passed for host_env, VM and stdlib. Threaded all-targets Clippy passed with exactly the same 12 existing diagnostics as the baseline and no new warning.
• Rustfmt, git diff --check, and normal commit hooks passed.

Native RustPython execution is not available under the required safety guard; no native runtime or full test-suite pass is claimed. The separate non-threaded/WASM graph was not built under the current disk limit.

Integration notes

The shared dictionary hash-error formatter extraction is intentionally identical to the helper in #9693 rather than another implementation. Its set operation changes still require reconciling add/add_known_hash: preserve #9693's hash-only contextualization, keep raw collision errors unchanged, and delegate to one raw insertion helper. The formatter also overlaps #9700's Unicode-preserving key_hash change: retain the extracted WTF-8 helper and #9700's other qualified-name improvements. Source composition was inspected; the combined branches have not been compiled.

Union/GenericAlias representation formatting and unrelated set-operation behavior remain outside this change. Repository tests, standard-library files and workflows are unchanged.