fix: bound MCP self-check pipe I/O and shutdown

[ejgallego]

This PR bounds protocol I/O and shutdown in MCP self-check when a server stops responding or a descendant retains its pipes. It builds on the diagnostic-logging correction in #276.

• Use one controller for nonblocking pipe I/O and child reaping, with absolute deadlines for writes, responses and teardown and an 8 KiB stderr tail on failure.
• Key native helper artifacts to the selected Lean toolchain to prevent cross-toolchain ABI reuse.
• Document the native helper assumption that Lean file handles wrap FILE pointers. The external lifecycle supervisor currently covers POSIX only; equivalent Windows lifecycle coverage remains outstanding.
• Keep deterministic typed Lake-helper failure and installed-MCP startup fixtures.