refactor(tui): simplify span merging and node ref helpers

[HarshK97]

Problem

1. Line highlight merging checked exact parent equality when deciding whether to join nearby edit spans. Child nodes under parent subtrees were treated as separate spans, causing fragmented highlight chips on screen.
2. Helper functions used manual loops and verbose switch statements for slice prefix comparison and operator checks.

What Changed

• TUI Highlight Merging: Added ancestor lineage checking so updates and moves within the same AST subtree merge into clean continuous highlight spans. Tightened maximum gap threshold from 5 to 3 columns.
• Node Ref Helpers: Replaced manual slice prefix and equality loops with Go standard library slices.Equal.
• Character Matching: Simplified operator character checks using bytes.ContainsAny.