v0.10.4

MCP release: barrel_mcp 4.3.0, and the body and keepalive options of
its own listener on livery_mcp.

Added

• livery_mcp options max_body_bytes (default 16 MiB, answered
  413), body_timeout_ms (default 60000, answered 408) and
  sse_keepalive_ms (falls back to the barrel_mcp app env). A body
  that failed to read used to reach the engine as an empty body.

Changed

• barrel_mcp ~> 4.3.0 in the test and check profiles. The MCP
  guide covers instructions, visible/4, authorize_subscribe/3
  and durable or application-hosted tasks, which work through livery
  unchanged.
• The per-chunk MCP body wait moves from 30s to 60s, as in barrel_mcp.