Fix Windows proxy startup and bounded native preface handling

[RobLe3]

Scope

Fix the seven failures reproduced by the Windows artifact builder: ASCII-safe proxy startup, bounded non-consuming native prefix classification without unsupported socket flags, and platform-correct fixtures. No protocol, public API, or release-version change. Experimental native transport remains outside the stable boundary.

Closes #134.

Validation

• Complete Windows diagnostic Python builder: PASS, including tests, packaging, online and offline installs; both expected artifacts emitted.
• Local pytest: 1,262 passed, two explicit skips, nine subtests passed.
• Ruff and checked mypy baseline: PASS (74/74 baseline findings unchanged).
• Workflow semantic preflight and portable-local lane: PASS.
• Temporary clean-HEAD structural comparison: no Sentrux degradation; no repository baseline replaced.

Qualification boundary

Diagnostic repair is not clean qualification. The affected source/artifacts must be refreshed after review and merge. Existing required checks remain intact; local results are not a substitute for the separately required clean Python 3.11 installation check. No publication or deployment.