- find -exec/-execdir children inherited the omp process's real
stdout/stderr, spamming output into the TUI terminal and bypassing
shell redirects; they also inherited the host env instead of the
shell's exported environment.
- Added pi_uutils_ctx::run_captured (moved from uu-xargs' private
helper): stdin null, stdout streamed into scope stdout, stderr
drained on a helper thread and forwarded after exit.
- uu-find exec matchers now use env_clear + env_snapshot and
run_captured; MultiExecMatcher rebuilds a std Command from the
argmax command's accumulated state (argmax only Derefs immutably).
- uu-xargs reuses the shared helper; added pi-shell regression test
asserting -exec child stdout flows through the shell redirect with
the exported env.
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