The grep and rg in-process builtins passed no-op heartbeats to pi_walker
when recursing into a directory, so the uutils scope's cancel flag was
ignored mid-walk. The shell wrapper sets that flag on abort/timeout and
then awaits the blocking task — with no heartbeat hookup, a cancelled
recursive grep/rg waited for the whole tree to be scanned before the
shell could return exit 130.
Add pi_uutils_ctx::is_cancelled() and have grep's search_dir, rg's
search_dir, and rg's collect_filtered_files supply heartbeats that
return io::ErrorKind::Interrupted when the flag is set. The walker maps
that to WalkError::Interrupted, which the utilities now silently treat
as a harness cancellation (no spurious 'native directory scan
interrupted' on the command's stderr — the shell wrapper owns the
user-visible exit code).
Regression tests pre-set the cancel flag and assert grep -r /
rg / rg --files exit without scanning the matching file in the tree.
Fixes#3933
roboomp
dbab0ef84d