feat: add interval literal syntax

[Seddryck]

Summary

• add first-class bounded interval literals with mathematical and reversed-square-bracket notation
• support infinite bounds and normalize the four existing sign-oriented shorthands
• expose normalized interval bounds and inclusivity through the C# syntax model
• add grammar corpus coverage and managed binding tests, including canonical temporal bounds and rejection of bare date-looking bounds

Validation

• Tree-sitter interval corpus: 5/5 passing against the generated parser
• dotnet test on net8.0: 78 passing
• dotnet test on net9.0: 78 passing
• dotnet test on net10.0: 78 passing
• git diff --check

Close #51

[coderabbitai]

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Run ID: eb33a2e5-0237-4ea0-8f7d-f14ad2a3a1a1

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