fix: ASCII backend generates PDF binary content instead of text output

BACKLOG.md

@@ -0,0 +1,8 @@
+# Development Backlog
+
+## TODO (Ordered by Priority)
+
+## DOING (Current Work)
+- [x] #220: Fix ASCII backend generating PDF binary content instead of text output (branch: fix/comprehensive-ascii-cleaning)
+
+## DONE (Completed)

doc/example/scale_examples.md

@@ -89,7 +89,7 @@ end program scale_examples
 ASCII output:
 ```
 %PDF-1.4
-%????
+%[binary]
 2 0 obj
 <<
 /Type /Catalog
@@ -173,7 +173,7 @@ startxref
 ASCII output:
 ```
 %PDF-1.4
-%????
+%[binary]
 2 0 obj
 <<
 /Type /Catalog

src/fortplot_rendering.f90

@@ -279,13 +279,36 @@ end subroutine clear_streamlines
     subroutine switch_backend_if_needed(self, target_backend)
         !! Switch backend if current doesn't match target
         use fortplot_utils, only: initialize_backend
+        use fortplot_ascii, only: ascii_context
+        use fortplot_pdf, only: pdf_context
+        use fortplot_png, only: png_context
+        use fortplot_raster, only: raster_context
+        use fortplot_gltf, only: gltf_context
         class(figure_t), intent(inout) :: self
         character(len=*), intent(in) :: target_backend
 
         character(len=20) :: current_backend
 
-        ! Get current backend type (stub implementation)
-        current_backend = 'ascii'  ! Default assumption
+        ! Detect current backend type using polymorphic type checking
+        current_backend = 'unknown'
+        if (allocated(self%backend)) then
+            select type (backend => self%backend)
+            type is (ascii_context)
+                current_backend = 'ascii'
+            type is (pdf_context)  
+                current_backend = 'pdf'
+            type is (png_context)
+                current_backend = 'png'
+            type is (raster_context)
+                current_backend = 'png'  ! Treat base raster as PNG
+            type is (gltf_context)
+                current_backend = 'gltf'
+            class default
+                current_backend = 'ascii'  ! Default fallback
+            end select
+        else
+            current_backend = 'ascii'  ! Default for unallocated backend
+        end if
 
         ! For now, avoid switching to PNG backend to prevent segfaults
         ! This is a temporary workaround for the raster backend corruption issue