Reduce diff allocations and fix line counting

[andrew]

Hunk lines in generateSimpleDiff and generateAddedDiff wrote each output line as prefix + line + "\n", two temporary strings per line (plus a []byte→string copy in the added-file path). Write the three parts directly to the builder.

countLines used bufio.Scanner, which stops silently at the first line over 64 KiB (minified JS, source maps) and returned a short count with no error. Replaced with bytes.Count plus a trailing-newline check; TestCountLines/long_line covers a 100 KB line that returned 0 before.

Compare accumulated TotalAdded/TotalDeleted only for modified files even though added files already reported LinesAdded. Deleted files now report LinesDeleted and both totals sum across all file types.

Adds BenchmarkGenerateAddedDiff (10k-line file) and BenchmarkGenerateSimpleDiff (5k lines, half changed). Interleaved benchstat, arm64 darwin, n=10:

                     │   before    │             after              │
                     │   sec/op    │   sec/op     vs base           │
GenerateAddedDiff      603.6µ ± 8%   440.3µ ± 6%  -27.05% (p=0.000)
GenerateSimpleDiff     307.8µ ± 8%   234.9µ ± 5%  -23.67% (p=0.000)
                     │  allocs/op  │  allocs/op   vs base           │
GenerateAddedDiff     10038.0 ± 0%    38.0 ± 0%   -99.62% (p=0.000)
GenerateSimpleDiff     2543.0 ± 0%    39.0 ± 3%   -98.47% (p=0.000)

[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Pull request overview

Improves diff generation performance, fixes long-line counting, and includes added/deleted files in aggregate totals.

Changes:

• Reduces allocations when generating diffs.
• Replaces scanner-based line counting.
• Adds coverage and benchmarks for updated behavior.

File summaries

File	Summary
diff/diff.go	Optimizes diff output and corrects line-count and aggregate-total logic.
diff/diff_test.go	Adds coverage for line counts, totals, and benchmarks.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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