feat(engine): add linediff engine and universal line diff fallback

[HarshK97]

Problem

1. Structural AST diffing can generate spurious matches across unrelated blocks when untouched lines pair up with edited nodes. So we need a fast line-level diffing engine to implement Matsumoto et al.'s two-group line partitioning strategy ("Beyond GumTree").
2. When tree-sitter cannot detect a file's language or fails to parse its AST, diffm previously failed with a hard parse error. So unsupported file formats and plaintext files couldn't be diffed.

What Changed

• Line Diff Engine: Added LineDiff(linesA, linesB) in internal/engine/linediff.go using a 1D DP flat slice and prefix/suffix line trimming.
• Universal Line Diff Fallback: Added BuildLineDiffEnvelope in internal/serialize/json.go and updated computeDiff in cmd/pipeline.go. When AST parsing fails or file extensions are unsupported, CLI and TUI fall back to line-level diffing instead of erroring out.
• TUI Deduplication: Replaced the local generateLineDiff helper in internal/tui/model.go to use serialize.BuildLineDiffEnvelope.
• Tests: Expanded unit tests in internal/engine/linediff_test.go for line trimming edge cases and added TestDiffFallbackLineDiff in cmd/diff_test.go.