fix(postprocess): use Src map for T1 node lookup in FilterPunctuation

[HarshK97]

Problem

1. When splitting punctuation move or update actions into delete and insert steps, the filter looked up the source node in ms.Dst().
2. ms.Dst() maps destination nodes to source nodes, so querying it with a source node returned nil. So the insert action for destination punctuation got silently dropped.

What Changed

• Punctuation Filtering: Fixed the map lookup in FilterPunctuation to use ms.Src(), so source nodes properly map to their destination counterparts and the insert action gets added.
• Unit Tests: Added tests covering strict punctuation move splitting, comma/semicolon filtering, and nil script handling.