fix: harden cleanup hints and purge empty artifacts

Harden uninstall scanning against PATH stat shims, avoid GUI-app-owned dotdir false positives, and add explicit zero-size purge artifact visibility.

Also relax the normalize_paths_for_cleanup performance regression test threshold to avoid CI runner timing noise while still catching O(n²) regressions.

Verification:
- ./scripts/check.sh --format
- MOLE_TEST_NO_AUTH=1 bats tests/uninstall_scan_bash32.bats tests/clean_hints.bats tests/purge.bats tests/completion.bats
- MOLE_TEST_NO_AUTH=1 bats tests/regression.bats
- go test ./cmd/...
- MOLE_TEST_NO_AUTH=1 MOLE_TEST_JOBS=2 BATS_FORMATTER=tap ./scripts/test.sh