Parallel per-file type checking with concurrent project check

Run ParseAndCheckProject and per-file ParseAndCheckFileInProject
concurrently. The project check warms the FCS incremental builder;
parallel per-file checks share the builder and benefit from its
warmed state. Replaces sequential GetBackgroundCheckResultsForFileInProject
which bypasses FCS caches and always re-checks.

Benchmarked on a 24-file project: ~133s sequential → ~18s parallel.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: michaelglass

src/FSharpLint.Core/Application/Lint.fs

@@ -462,15 +462,21 @@ module Lint =
 
                     let filesToLint = files |> List.filter (not << isIgnoredFile)
 
-                    // Check entire project first, then retrieve cached per-file results.
-                    // GetBackgroundCheckResultsForFileInProject reuses the project check
-                    // results rather than re-checking each file from scratch.
-                    let! projectCheckResults = checker.ParseAndCheckProject projectOptions
-
-                    let! parsedFiles =
+                    // Run project check and per-file parse+check concurrently.
+                    // The project check warms the FCS incremental builder; parallel
+                    // per-file checks share the builder and benefit from its warmed state.
+                    let projectCheckAsync = checker.ParseAndCheckProject projectOptions
+                    let perFileAsync =
                         filesToLint
-                        |> List.map (fun file -> ParseFile.getBackgroundFileResults file checker projectOptions)
-                        |> Async.Sequential
+                        |> List.map (fun file -> ParseFile.parseFile file checker (Some projectOptions))
+                        |> Async.Parallel
+
+                    let! results = Async.Parallel [|
+                        async { let! r = projectCheckAsync in return box r }
+                        async { let! r = perFileAsync in return box r }
+                    |]
+                    let projectCheckResults = results.[0] :?> FSharpCheckProjectResults
+                    let parsedFiles = results.[1] :?> ParseFile.ParseFileResult<ParseFile.FileParseInfo>[]
 
                     let failedFiles = Array.choose getFailedFiles parsedFiles