Complete Python WinRT projection readiness

[leileizhang (lei9444)]

Summary

• harden Python WinRT scalar, array, struct, HRESULT, async-progress, datetime, and nullable-object boundaries
• make Python codegen more idiomatic and deterministic with native constructors, wrapper identity caching, value-style structs, UInt naming, and overload dispatch
• expand Python/runtime/codegen regression coverage, cross-language E2E checks, snapshots, and enforce calibrated coverage gates

Validation

• cargo test -p dynwinrt -p dynwinrt-codegen --quiet
• Python binding build, pytest, and mypy.stubtest
• Python E2E: 41/41
• TypeScript E2E: 37/37
• coverage threshold fixture tests
• mixed-language coverage pipeline (-SkipCom): Rust 46.81%, Python 71.2%, JavaScript 22.21%

Remaining validation

• the live CPython 3.11-3.14 ARM64 release matrix requires the external self-hosted ARM64 runner
• full-workspace cargo fmt --all --check remains blocked by pre-existing formatting in benchmark files outside this diff and will be handled separately

[github-actions]

Mixed-language test coverage

Workflow status: ✅ Passed

Layer	Lines	Functions	Branches/regions
Rust, including native .pyd/.node	81.48%	77.88%	81.39% regions
Generated Python projections	71.2%	n/a	36.54% branches
JavaScript aggregate	18.77%	20.83%	52.26% branches
JavaScript runtime	19.91%	14.28%	7.14% branches
Generated WinRT projections	22.27%	20.02%	52.85% branches
Generated Classic COM projections	12.97%	23.45%	52.17% branches

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