fix(postprocess): enforce container provenance in stationary move check

[HarshK97]

Problem

1. When deletions shift lines up, an item moved into a different container can land on the exact row and column where an old element used to sit.
2. isStationaryMove only checked coordinates within the enclosing function. Because the line and column numbers matched, it assumed the node never moved and dropped the move highlight entirely.

What Changed

• Check container provenance before treating matching coordinates as stationary. The destination container actually has to map back to an old container that held this node (and vice versa). If they're distinct containers, we keep the move.
• Reorder checks in isStationaryMove so direct parent checks and stationary expression moves are evaluated in the right order.
• Add a regression test in internal/postprocess/normalize_test.go where an item moves between two slice literals and lands on the exact same row and column coordinates.