Replace ArrayPool with direct allocation and improve line terminator handling

[danipen]

Summary

• Replace ArrayPool<char> with direct char[] allocation when appending newlines in Grammar.Tokenize(). ArrayPool buffers can be reused with different content, which breaks Onigwrap's regex search caching (uses memory reference equality). Direct allocation creates unique buffers that enable proper caching, improving overall tokenization performance.
• Update Demo program to pass lines with line terminators included, using ReadOnlyMemory<char> slices from a single file read. This avoids per-line allocations and demonstrates the optimal usage pattern.
• Add XML documentation to IModelLines and ITMModel interfaces, including guidance that GetLineText() should return lines with terminators to avoid internal allocations.

⚠️ Breaking Changes

• IModelLines.GetLineText() was renamed to IModelLines.GetLineTextIncludingTerminators() for API clarity.