Add comprehensive tests for the new graph.go functions to meet
coverage threshold after rebase introduced 405 new lines:
- TestTruncateTitle: tests rune-safe title truncation
- TestPadRight: tests rune-safe string padding
- TestRenderNodeBox: tests ASCII box rendering for all status types
- TestComputeLayout: tests topological layout computation with
dependencies
These tests cover the pure utility functions and basic graph layout
logic, bringing coverage from 44.7% to 45.1%.
