Fix Windows renderer staging ACL inheritance

[kingbentley]

Summary

• create renderer staging directories with inherited ACLs on Windows
• preserve tempfile.mkdtemp() and private 0700 behavior on non-Windows platforms
• add focused tests for platform selection, entropy, collision retries, exhaustion, and unexpected filesystem errors
• register the new tests in the existing test:renderer quality path

Root cause

Python 3.11.10 and newer specially handle mkdir(..., 0o700) on Windows by creating a protected ACL for the owner, SYSTEM, and administrators. tempfile.mkdtemp() uses that mode. In a restricted sandbox, the protected ACL drops inherited workspace principals, so the renderer can create its staging directory but is denied when writing the first snapshot file.

Impact

Sandboxed Windows rendering can publish immutable workflow snapshots without requiring an unsandboxed workaround. Existing atomic rename, concurrent-winner verification, cleanup, destination layout, and POSIX security behavior remain unchanged.

Validation

• npm ci
• npm run quality under WSL with Node 22.23.2 and uv 0.12.4 — passed
• renderer suite within quality: 15/15 Python tests and 24/24 integration tests passed
• consumer-project sandbox verification: renderer completed and the emitted workflow.md was readable from a separate sandboxed process