Returns the path to an executable esbuild sidecar sitting next to execDir, or null if none is present. On Windows the .exe variant is preferred. Pure and injectable so the lookup order is unit-testable without a real compiled binary.

findSidecarEsbuild('/tmp/no-sidecar-here'); // null — nothing executable next to the binary
findSidecarEsbuild('/tmp/no-sidecar-here', 'win32'); // null — checked esbuild.exe first