Adopt curl-compatible file output

[unitmatrix]

Summary

• stream -o FILE directly with normal overwrite and truncation semantics
• treat -o - as standard output and preserve normal symlink, FIFO, and device behavior
• retain partial named output after late extraction failures
• update README and zget(1), with focused regression coverage

Tests

• ctest --test-dir build --output-on-failure
• ctest --test-dir build-stage4-clang --output-on-failure
• ASAN_OPTIONS=strict_string_checks=1 UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1 ctest --test-dir build-stage4-sanitize --output-on-failure