Handle missing terminfo fallbacks and restore unicode test

[assisted-by-ai]

Summary

• restore the unicode test hard failure when the trojan-source checkout is absent
• add TERM-based fallbacks for get_sgr_support so stdisplay handles missing terminfo entries without spurious failures

Testing

• ./run-tests (fails: pylint not available in environment)

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Codex Task

[assisted-by-ai]

Added TERM-aware fallbacks in get_sgr_support so stdisplay handles missing terminfo entries gracefully while still enforcing conservative color support where appropriate.

[ArrayBolt3]

Rejected. The heuristics are really not a good idea IMO. We have terminfo, if terminfo is broken falling back to no color is safest. If the user really wants color support despite their broken or otherwise unsupported setup, they can export COLORTERM=truecolor.