[codex] refactor(vt): move color encoding to internal vt

[tonyfettes]

Summary

• Move semantic color-to-SGR byte encoding from root style.mbt into internal/vt.
• Keep root Tty color command methods as the public API and delegate byte generation to internal/vt.
• Move public Tty behavior coverage from tty_wbtest.mbt to black-box tty_test.mbt, leaving private State invariants in white-box tests.

Validation

• moon fmt
• moon test internal/vt
• moon test .
• moon test
• moon check
• moon info
• git diff --check