Keep experimental native TCP outside stable defaults

[RobLe3]

Summary

• require explicit provider/runtime opt-in before mounting or advertising native TCP
• keep directory-advertised native endpoints out of normal client routing unless the experimental binding is enabled
• document plaintext native TCP as development-only and outside the stable/production baseline

Validation

• full local quality matrix on Python 3.11, 3.12 and 3.13
• 1,242 tests passed with 2 skips per runtime
• Ruff, mypy baseline, audit, build and clean-install gates passed
• coverage 76.177% (minimum 75%)

No release or deployment is part of this PR.