• Select ACP v2 for new connections when the existing acp-beta feature
  flag is enabled; leave flag-off connections on v1 and preserve v1 beta
  behavior on fallback.
• Start a fresh v1 process/transport only after a correlated
  initialization exchange establishes v1-only support: a valid v1
  initialization response or structured -32022 data with requested: 2
  and a well-formed supported list containing 1 but not 2. Do not guess
  from error-message text or retry malformed initialization, unrelated
  protocol errors, or ordinary transport failures.
• Drain initialization output on natural process exit before classifying
  failure, preserving structured negotiation diagnostics even when the
  agent exits immediately after rejection.
• Include v2 connections in the existing ACP logs UI without adding
  debug-log requirements to the generic connection trait.

This is behavior-changing activation for the existing acp-beta
audience.

Release Notes:

• N/A