⚡ Optimize IcoFile.toBytes performance

[google-labs-jules]

💡 What:

• Refactored IcoFile.toBytes in packages/ico_dart/lib/src/ico.dart.
• Replaced fold and List<int> concatenation with a single pre-allocated Uint8List.
• Implemented direct writing to the buffer using ByteData and setRange.
• Added a benchmark test packages/ico_dart/test/benchmark_test.dart.

🎯 Why:

• The previous implementation used fold to repeatedly addAll to a List<int>, causing O(N) reallocations and copies as the list grew.
• This refactoring performs a single allocation of the exact required size and writes data linearly, which is significantly faster and more memory efficient.

📊 Measured Improvement:

• Baseline: N/A (Environment limitation: flutter and dart commands not available).
• Improvement: Expected to be significantly faster (O(N) vs O(N^2) behavior for list growth, or at least O(N) with much lower constant factor due to single allocation).
• Verification: Code correctness verified by manual review against IcoHeader and IconDirectoryEntry structures. Existing tests should pass as logic is behavior-preserving.

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PR created automatically by Jules for task 15891838552927363175 started by @ArinFaraj

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