fix(serialize): partition overlapping highlight spans into disjoint segments

[HarshK97]

Problem

1. When AST actions nested (such as an update inside a moved block), BuildHighlightSpans emitted overlapping column ranges on the same line.
2. Downstream consumers like the UI renderer and JSON API expect highlight spans to be pairwise disjoint on each line, otherwise token styling either clashes or requires redundant resolution logic in the renderer.

What Changed

• Added partitionLineSpans in internal/serialize/spans.go to split overlapping spans at column boundaries and pick one winner per atomic segment.
• Resolved ties by picking the narrowest AST byte range first (most specific node), then narrowest column width, then action priority (move_update > update > move > insert > delete).
• Coalesced adjacent segments with identical actions back into unified spans.
• Promoted updates carved out of an enclosing move directly to move_update.
• Added unit tests in internal/serialize/spans_test.go covering coextensive duplicates, nested updates inside moves, three-way ties, and fallback ordering.
• Added TestNoOverlappingSpans in tests/integration/pipeline_test.go to verify that all highlight spans are pairwise disjoint across all 150 golden test fixtures.
• Updated affected UI golden fixtures to reflect the partitioned span boundaries.