Improve ANTLR parser handling

[yetanotherchris]

Summary

• parse templates once and cache the parse tree in Template
• add SLL parsing with LL fallback in TemplateEngine
• expose helpers for processing cached parse trees
• update benchmark results in README

Testing

• dotnet test --verbosity minimal
• dotnet run -c Release --project benchmarks/TextTemplate.Benchmarks -- --job Short --filter "*" (5 runs)

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https://chatgpt.com/codex/tasks/task_e_684eeddc5648832fbd1e8b64fbcee5f7