fix(cli): render Unicode output with legacy encodings

[Bortlesboat]

Summary

On Windows and redirected streams configured as CP1252, the CLI could raise UnicodeEncodeError before showing its banner or graph summaries because those views use box-drawing characters. Reconfigurable standard streams now switch to UTF-8 at startup, while UTF-8, encoding-less, and non-reconfigurable streams keep their existing behavior.

Validation

• Reproduced the crash on clean main with a CP1252 TextIOWrapper before applying the fix.
• The real main() regression passes on Python 3.10 and 3.13 and verifies both stdout and stderr.
• tests/test_cli.py: 18 passed; the one deselected test is an existing Windows failure reproduced on clean main.
• Full Windows suite: 2,353 passed; 37 unrelated platform or fixture failures, with representative cases reproduced on clean main.
• Ruff, targeted mypy, and the configured Bandit scan pass.
• Real processes launched with PYTHONIOENCODING=cp1252 render both the startup banner and graph summaries and exit successfully.

Post-Deploy Monitoring & Validation

No additional operational monitoring is required because this changes local CLI output and has no deployed service surface. During review and the next Windows release, a healthy run exits successfully and renders readable Unicode when PYTHONIOENCODING=cp1252; UnicodeEncodeError, charmap, or unreadable redirected output are failure signals. Revert this commit to restore the previous stream behavior if those signals appear.

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