[GreedyC]

Summary

Closes #114.

The existing BrokenPipeError handler catches drawing failures, but buffered output can still fail when Python flushes stdout during interpreter shutdown. A closed pipe then prints an ignored exception and exits with code 120.

Run the complete CLI, including argument parsing and early exits, inside the protected block and flush stdout in a finally block. On a broken pipe, redirect its descriptor to os.devnull, close the extra descriptor, and exit with code 1. This follows the Python documentation's SIGPIPE guidance, and preserves normal output.

Validation

• Four real subprocess/pipe regressions cover small and large charts, --version, and --help. The two additional early-exit cases fail against the initial chart-only fix and pass after moving the full CLI into the protected block. Initial chart regressions: a small buffered chart and a large chart. Both fail against the old production code with code 120 and stderr; both pass with exit 1 and empty stderr after the fix.
• Full suite: 31 passed on Python 3.14 and the minimum supported Python 3.9.
• A normal CLI run still succeeds and prints its chart.
• Mypy: no issues in eight source files. Source and wheel builds pass.
• Repository-wide Ruff currently reports 65 existing diagnostics; the filename/code/message multiset is identical with the production fix reverted. The new tests pass their own Ruff check. This is not a clean repository-wide lint result.
• git diff --check passes; Copilot pre-review found no introduced defect.

Validated on macOS arm64; Windows execution was not tested. No signal-handler changes or dependencies are introduced.