Add size limits for large diffs in comparison reports

[ArjenSchwarz]

Context

From code review of #21 (multi-spec comparison feature).

Current Behavior

Large diffs are split into separate files when they exceed 500 lines (internal/report/generator.go:78-92), which is good. However, the threshold is line-based only.

Potential Issue

Very long lines or binary content could still result in large memory usage even with fewer than 500 lines.

Suggested Improvements

1. Add a byte size check in addition to line count (e.g., 100KB threshold)
2. Add warnings when processing very large repositories
3. Consider streaming for extremely large diffs

Priority

Low - edge case that would require unusually large single-file changes to trigger.