fix(engine): prevent large-distance line jumps in LineDiff DP backtrack

[HarshK97]

Problem

1. In large JSON schema files like json_schemastore_workflow_step_reorder, identical lines like "$comment" or "type": "string" appear inside multiple definition blocks 50 to 200 lines apart.
2. During DP backtrack, LineDiff greedily picked matching lines regardless of line distance. So line partitioning jumped across distant schema blocks, locking candidate AST matches across unrelated definitions and producing 42 actions (31 of them spurious moves) for a simple definition reorder.

What Changed

• Updated DP backtrack in internal/engine/linediff.go to inspect line distance offsets.
• When the default backtrack target j is more than 20 lines away (i-j > 20 or j-i > 20), LineDiff looks for an alternative optimal DP line k within a 20-line local window.
• This keeps contiguous sequence alignment intact for local code blocks while stopping line partitions from jumping across distant blocks in large schema configs.
• Updated tests/testdata/json_schemastore_workflow_step_reorder/expected.json

Visual Comparison

• Example : tests/testdata/json_schemastore_workflow_step_reorder/

Before

• Spacious moves are generated

After

• No spacious moves are generated, and diff is clean.