Text truncation now returns head/tail parts rather than a pre-joined ellipsis string

The truncation helper now returns { head, tail } objects instead of a pre-joined ellipsis string, with callers responsible for joining them. The renderer routes truncate-start, truncate-middle and middle modes through this new path. The measured-line class also precomputes displayText and displayStartOffset so highlight offsets are computed once rather than being re-trimmed on every row.