Apply the 64 MiB protocol message limit when connect callers omit an override. Reject invalid explicit limits, including values that wrap through ws signed 32-bit coercion and disable enforcement. Preserve the typed payload_limit reason through disconnect and later commands.
Validation: 16 transport boundary tests passed, including UTF-8 exact/+1 limits and an oversized frame header without allocating its body. Real Chrome 154 CLI/MCP browser verification and check:fast passed. This caps message bytes, not decoded JSON heap size.