Avoid duplicate AST initialization and lazy-import locals allocation - #9183

Summary

Follow up on the two review findings in #9179 after its merge.

• Let PyType::call invoke the AST initializer once. NodeAst::slot_new only allocates, matching CPython's PyType_GenericNew. This also preserves custom subclass initializers that deliberately omit super().__init__() and allocation-only direct __new__ calls.
• Check existing frame-locals identity without materializing a fresh locals dictionary during ordinary eager imports. Keep the module-level check before the __lazy_modules__ lookup.

Only two Rust files change. No library copies, test expectations, or skip markers change.

Validation

Fresh release binary built from this follow-up on main 9f6f7719.

• Eight bounded observations exactly match pinned CPython 3.15 d220409: abstract warning count, inherited field writes, custom/delegating initialization, and direct __new__ allocation.
• Full canonical test_ast: 220 total, 205 pass, 9 skip, 6 existing expected failures.
• Full canonical test_lazy_import: 161 total, 157 pass, 4 skip.
• Corresponding pinned-CPython oracle suites pass.
• Fresh workspace native tests: 1,373 pass across 22 targets.
• Fresh C-API native tests: 123 pass, 4 existing ignored tests.
• Strict workspace and C-API Clippy pass with -D warnings.

The existing missing-C-API skips and AST expected failures remain visible; these results do not claim complete AST or compiler parity.