Add unit tests and CI workflow

[adibsaad]

Summary

• Unit tests (cargo test, 18 passing) covering the pure logic and UX invariants:
  • proc_name basename extraction + fallback, parse_cpu_time (BSD + procps formats)
  • matches multi-term AND + smart-case; basename/pid-only search semantics
  • build_children, walk_tree (exact connector rendering), build_search (cycles safe)
  • filtering shows matches with ancestors as context, cursor lands on first killable row and clamps at bounds
  • protected rows refuse selection; selection view lists selected + ancestors and self-exits when empty
  • kill_targets skips protected selections and falls back to the cursor row
• Applied cargo fmt (mechanical reformat)
• CI workflow: fmt --check + clippy -D warnings + test on ubuntu and macos, for pushes to main and all PRs

Deliberately untested

• fetch() (shells out to live ps) and do_kill() (fires real SIGKILLs)