[cppla]

Summary

• Retry a shared web TCP/UDP :0 allocation only when the selected UDP port is already occupied, up to eight total attempts. A fixed port on either side, any other bind error, and any rollback failure still fail immediately.
• Preserve non-benign cleanup failures inside joined/wrapped errors instead of treating the entire error as already closed.
• Add deterministic real-socket collision/rollback tests, retry-policy and cleanup-error regressions, explicit non-skipped Windows repetitions, and protocol documentation.

Validation

• Final source: GOFLAGS=-p=1 make check and GOFLAGS=-p=1 make race passed.
• Focused new/normalizer tests: five top-level tests, three repetitions each on macOS; no failures/skips.
• Isolated, network-disabled Linux ARM64 Docker: eight top-level tests, three repetitions each; no failures/skips.
• Same SHA256-verified test binary on the native ARM64 test host: the same eight tests, three repetitions each; no failures/skips. Process exited; temporary evidence retained, no services deployed.
• GitHub checks: 14/14 passed for 1ab4e351f23d8bdc7d6f84acb5e91e249669334f, including Windows explicit three-pass/zero-skip assertions for each new listener-retry test.
• Independent code/test review found no merge blockers.

Scope

This addresses an observed ephemeral TCP/UDP port collision from the preceding upgrade validation. It does not change fixed deployment endpoints, authentication, handshake profiles, or wire behavior. No large passive corpus, comparative claim, version tag, or release is included.

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