Test server with controllable tool timing for async dispatch tests.

Tools:

• wait_signal — sends {:tool_running, self(), signal} to frame.assigns[:test_pid], then blocks on receive {:proceed, ^signal}. Lets tests inspect mid-flight state.
• increment — reads/writes frame.assigns[:counter]. Verifies FIFO frame ordering.
• crash — raises. Verifies async crash isolation.
• echo — returns the input verbatim. Verifies session survives a previous crash.