* Sync site startup processing and tests with CPython 3.15
Preserve existing RustPython identity, path expectations, and four _pth expected failures via explicit updater metadata.
Assisted-by: OpenAI Codex: model version unavailable
* Honor standalone executable ._pth startup configuration
Apply explicit path isolation before CLI environment settings, preserve canonical empty-file and prefix handling, and retire three expected failures proven by test_site.
Assisted-by: OpenAI Codex: model version unavailable
* Retire passing Windows underpth expectation
The canonical DLL-named test passed in both Windows CI attempts on e1527c53. Remove only its obsolete expectedFailure through update_lib.
Assisted-by: OpenAI Codex: model version unavailable
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Co-authored-by: Codex <codex@openai.com>