Improve lexer input buffering and Unix generator builds - #238

Summary

• reduce deferred input-buffer compaction overhead
• use a circular integer queue while preserving the EOF sentinel
• run Unix generators through dotnet with major-version roll-forward fallback

Validation

• net8.0 build passed
• dotnet test Test/SqlDom/UTSqlScriptDom.csproj -f net8.0: 1205 passed
• 5 existing SYSLIB0051 warnings remain

The generated-source changes are build outputs only and are not included.