fix: terminate cleanly on a broken stdout pipe across all family-emitting commands

GREEN for the broken-pipe RED test (c750b9f). Rust's runtime sets SIGPIPE to
SIG_IGN at startup, so a println! to a closed stdout pipe returned EPIPE and
panicked ("failed printing to stdout: Broken pipe", exit 101). Restore the
default SIGPIPE disposition once at the top of main() (cfg(unix), via
libc::signal) so a closed pipe terminates the process cleanly on SIGPIPE
(signal 13 / exit 141 — the conventional Unix CLI outcome, e.g. `yes | head`)
instead of panicking. One process-level reset covers every family-emitting
subcommand (sweep / mc / walkforward / runs family / generalize); the print
sites are untouched (minimal fix).

libc added as a direct aura-cli dependency — already transitive in Cargo.lock,
the standard vetted crate for signal disposition, admitted under the per-case
dependency policy.

Verified by the orchestrator: the broken-pipe test passes 6/6; `aura sweep | head`
exits 141 with no panic in stderr; cargo test --workspace green (0 failed);
clippy --workspace --all-targets -D warnings clean.

refs #146

Cargo.lock

@@ -57,6 +57,7 @@ dependencies = [
  "aura-registry",
  "aura-std",
  "data-server",
+ "libc",
  "serde",
  "serde_json",
 ]