fix(v2): enforce initialization lifecycle

[benbrandt]

Summary

• require initialize to be the first protocol message and allow it exactly once per experimental v2 connection, including the standalone batch form
• queue built-in, extension, and compatibility traffic while initialization is in flight, and reject queued work when negotiation or the connection fails
• expose the validated initialize exchange through one initialized promise on connection handles and handler contexts while preserving onConnect timing
• require v2 agents to register an initialize handler, preserve peer initialization during direct teardown, and close persistent clients after terminal negotiation failures

Verification

• npm run generate
• npm run check (40 files, 886 tests)
• npx vitest run src/v2/acp.test.ts (23 tests)
• direct lifecycle stress runs for immediate close, rejected callbacks, and throwing batch mappers
• git diff --check