perf: simplify render_text with one growable canvas per height - #19

Bugbot Analysis Progress (7m 30s elapsed)

✅ Gathered PR context (1s)
✅ Completed bug detection (7m 28s)
✅ Posted analysis results (0s)

Final Result: Bugbot completed review - no issues found! ✅

Request ID: 912aa29b-ba55-4c43-8345-6a96d815776c