Perf: reduce allocations and redundant work in hot paths

[Thorium]

Performance improvements:

• FileSystem.fs: Cache Lines property on RoslynSourceTextFile via lazy, avoiding repeated Seq.toArray + Array.map on every access. Replace ToString().GetBytes() in GetFileContent with chunked ISourceText.CopyTo to avoid materializing the entire source as an intermediate string. Uses UTF8Encoding(false) to avoid BOM, matching original behavior.

• Lexer.fs: Replace Array.fold with a simple for-loop for parsing defines and lang version from args. Each call creates its own FSharpSourceTokenizer (they are lightweight); no shared mutable state across threads.

• CompilerServiceInterface.fs: Move normalizePath into each match branch of SourceFilesTagged to avoid an extra List.map pass. Replace quadratic Array.append fold in processFSIArgs with ResizeArray for O(n) behavior.

• AdaptiveServerState.fs: Cache SourceFilesTagged on LoadedProject via a Lazy field and factory method, avoiding repeated List.map + List.toArray on every access. Replace OTel tag 'source.text' (which boxed entire file contents as a string) with 'source.length' (just the int length).

• AdaptiveFSharpLspServer.fs: Add rereadFile parameter to completion retry logic so the file is only re-read when the error indicates stale content (e.g. line lookup failure, trigger char mismatch), not on every retry. The 'empty completions' case skips re-read since only typecheck matters.