Warn about generated Python Windows output path lengths

[leileizhang (lei9444)]

Problem

Rust can emit generated Python files that ordinary Windows Python imports, mypy, or pip cannot open. The existing 120-character module-component budget cannot constrain an arbitrary output root.

Fix

Emit one actionable stderr warning before transactional publication, using the actual final generated .py/.pyi inventory, including namespace facades. Report the longest representative path, its UTF-16 length, and the shorter-output budget at the legacy 260-unit boundary. Resolve the caller's logical absolute path without counting extended prefixes, staging directories, or a junction's longer target. Dry runs aggregate all selected namespaces into one warning without writing; --no-pyi excludes stubs.

Document preferred short output/checkouts and validated same-path junction/subst recipes, with primary Windows/Python references. Generation still succeeds; generated names, imports, layout, bytes, manifests, runtime ABI, versions, and machine configuration are unchanged.

A separate additive CI-only prerequisite carries the exact authorized patch from 49c7e6e2e7042b527f1fee9d6655a8df8f537099: accept trailing commas in the existing test-hook stub! macro and format two calls for hosted Rust 1.99. Native signatures and behavior are unchanged; no functional feature prerequisite is imported.

Verification

cargo fmt --all -- --check and the original 57 focused Rust checks passed. Follow-up passed 4 CLI regressions, 7 path/classifier tests and 6 snapshots, including eight types across two namespaces yielding exactly one dry-run warning and no filesystem writes. cargo check -p jswinrt_rs --features test-hooks --lib passed for both ARM64 and x64. One additional JS-dependent naming check was unavailable without the built bindings\js\dist\winrt.js runtime.

Native Windows ARM64 / CPython 3.13.15 / LongPathsEnabled=0, using an isolated runtime wheel built from this main-based checkout:

Consumer path	Longest .py / .pyi	Result
Deep output	261 / 262	Codegen exits 0 with one warning; import, strict mypy, package build fail
Short output	159 / 160	No warning; 147 modules import, strict mypy and package installation pass
Same-byte junction	161 / 162	No warning; import, strict mypy and installation pass
Same-byte subst	145 / 146	No warning; import, strict mypy and installation pass

All 303 generated files are byte-identical to unchanged main and across these paths. The documented relative-output junction recipe also passed generation/import/mypy/install.

Limitations

Diagnostic only, not an automatic long-path guarantee. Pip build staging and deep venv/install destinations cannot all be predicted from codegen output. A short-built identical wheel still fails in a deep venv. The existing test_generated_python_long_paths.ps1 package build also fails identically on unchanged main in this long-path-disabled environment; this PR does not claim to fix it. No registry/TLS changes, module-layout redesign, or functional unmerged-branch dependencies.

[github-actions]

Mixed-language test coverage

Workflow status: ✅ Passed

Layer	Lines	Functions	Branches/regions
Rust, including native .pyd/.node	86.91%	82.48%	86.64% regions
Python aggregate	72.34%	n/a	39.42% branches
Python runtime	98.17%	n/a	94.57% branches
Generated Python WinRT projections	71.07%	n/a	33.5% branches
Generated Python WinRT implementations	71.97%	n/a	46.3% branches
JavaScript aggregate	21.91%	25.18%	57.61% branches
JavaScript runtime	44.27%	45.76%	78.99% branches
Generated WinRT projections	22.8%	18.7%	54.84% branches
Generated WinRT implementations	45.99%	59.19%	60.97% branches
Generated Classic COM projections	11.88%	23.97%	53.4% branches

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