Windows official Pi smoke cannot complete

Status: open; deferred outside the ThinkingCap model campaign.

The required repository test run stopped with 7,923 passed, 417 skipped and one failure in tests/workbench/test_pi_official_smoke.py. Independent reproduction identified two Windows problems before any model request:

1. The fixture replaces the child environment and drops SystemRoot. Pinned Node 22.23.2 aborts with status 134 at ncrypto::CSPRNG(nullptr, 0). Adding only SystemRoot fixes the isolated crypto probe.
2. With that variable restored, Pi returns valid RPC output, but PiRpcClient._default_read_chunk uses select.select on an anonymous pipe. Windows rejects that operation with WinError 10093; the client swallows it as empty output and the test times out.

The exploratory fixture-only change was reverted because it was insufficient. The model campaign leaves both runtime and test sources unchanged and does not claim a passing repository-wide test gate.

Fix the Windows pipe reader with a bounded platform-specific path, preserve the POSIX path, retain bounded child stderr on unexpected exit, and preserve SystemRoot in the Windows test environment. First pass the real pinned Pi smoke, then the full test wrapper. Do not replace the real smoke with a mock or skip it to obtain a green gate.

Evidence: docs/findings/2026-09-23-thinkingcap-evidence/validation-pi-failure.json.