The process-wide uncaughtException handler logged the error and kept running. For
the detached `serve --mcp` daemon that turned any escaped fault into an
unrecoverable orphan: nothing respawns it, and when logging the raw Error hit a
V8 source-position loop while lazily formatting `.stack`, the main thread wedged
at 100% CPU so even the PPID watchdog / idle-timer could no longer fire. Same
failure mode as #799, which only fixed the stdin-'error' trigger.
Restore Node's default fatal semantics: render a bounded, hang-proof line (name +
message only — never read `.stack`) then exit non-zero, so a fresh daemon starts
on the next connection. Extracted to src/bin/fatal-handler.ts with injectable
seams; unit-tested incl. the never-touch-stack invariant.
Closes#850.
