Never create more checkers than files in a program

[jakebailey]

Anything more is just waste.

May be useless in the future if we start treating checkers as an external resource to a Program (and thus the limitation would be put elsewhere if at all).

[Copilot]

Pull request overview

This PR optimizes checker pool creation by ensuring the number of checkers never exceeds the number of files in a program. The change moves checker pool initialization to occur after file processing is complete, allowing the pool size to be capped at the file count.

• Moved initCheckerPool() call to after processAllProgramFiles() in NewProgram
• Added finishedProcessing flag to enforce initialization ordering
• Updated newCheckerPool() to limit checker count to min(max(checkerCount, 1), len(program.files), 256)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
internal/compiler/program.go	Reordered initialization to call initCheckerPool() after processAllProgramFiles(), and added panic check to enforce this ordering
internal/compiler/fileloader.go	Added finishedProcessing flag to processedFiles struct, set to true when file processing completes
internal/compiler/checkerpool.go	Moved checker count logic into newCheckerPool() and added constraint to limit checkers to the number of files

[jakebailey]

This will conflict with #2159 so I'll wait for that as it's more critical.