fix(sacred): balanced ternary format — all tests pass

- Fixed buffer lifetime bug: use caller's buffer directly
- Fixed negative number handling: invert 1↔T after conversion
- Fixed test expectations: corrected math for 16 (1TT1) and -5 (T11)
- All 18 tests now pass

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Antigravity Agent