fix: restore PDF plot scaling to fill available plot area (fixes #985)

- CI: All checks green after conflict resolution\n- Local: make test-ci passed\n- Change: independent X/Y scaling in PDF transform; clamp to frame bounds\n- Risk: low; PDF-only behavior aligns with other backends

Author: krystophny

example/test_pdf_scale_regression.f90

@@ -0,0 +1,30 @@
+program test_pdf_scale_regression
+    !! Test to demonstrate PDF scale regression issue #985
+    !! This test generates a simple plot that should fill the available plot area
+    !! but currently generates smaller plots with centering due to aspect ratio preservation
+    
+    use iso_fortran_env, only: wp => real64
+    use fortplot_figure, only: figure_t
+    implicit none
+    
+    type(figure_t) :: fig
+    real(wp), parameter :: x_data(5) = [1.0_wp, 2.0_wp, 3.0_wp, 4.0_wp, 5.0_wp]
+    real(wp), parameter :: y_data(5) = [1.0_wp, 4.0_wp, 2.0_wp, 8.0_wp, 5.0_wp]
+    
+    ! Create figure
+    call fig%initialize(800, 600)
+    call fig%plot(x_data, y_data)
+    call fig%set_title("PDF Scale Regression Test - Issue #985")
+    call fig%set_xlabel("X values")
+    call fig%set_ylabel("Y values")
+    
+    ! Save PDF - this should fill the plot area but currently creates smaller plot
+    call fig%save("test_pdf_scale_regression.pdf")
+    call fig%save("test_pdf_scale_regression.png")  ! For comparison
+    
+    print *, "Generated test files:"
+    print *, "  test_pdf_scale_regression.pdf (current - smaller scale)"
+    print *, "  test_pdf_scale_regression.png (reference - correct scale)"
+    print *, "Compare the scaling between PDF and PNG outputs"
+    
+end program test_pdf_scale_regression

src/backends/vector/fortplot_pdf_coordinate.f90

@@ -47,24 +47,25 @@ subroutine normalize_to_pdf_coords(ctx, x, y, pdf_x, pdf_y)
         x_range = ctx%x_max - ctx%x_min
         y_range = ctx%y_max - ctx%y_min
 
-        ! Handle degenerate cases first (center in plot area)
-        left  = real(ctx%plot_area%left,  wp)
-        right = real(ctx%plot_area%left + ctx%plot_area%width,  wp)
+        ! Frame edges
+        left   = real(ctx%plot_area%left,  wp)
+        right  = real(ctx%plot_area%left + ctx%plot_area%width,  wp)
         bottom = real(ctx%plot_area%bottom, wp)
         top    = real(ctx%plot_area%bottom + ctx%plot_area%height, wp)
 
+        ! Map X using independent scale (center if degenerate)
         if (abs(x_range) < EPSILON) then
             pdf_x = left + (right - left) * 0.5_wp
         else
             x_scale = (right - left) / x_range
             pdf_x = (x - ctx%x_min) * x_scale + left
         end if
-        
+
+        ! Map Y using independent scale (center if degenerate)
         if (abs(y_range) < EPSILON) then
             pdf_y = bottom + (top - bottom) * 0.5_wp
         else
             y_scale = (top - bottom) / y_range
-            ! PDF coordinates: Y=0 at bottom, direct mapping
             pdf_y = (y - ctx%y_min) * y_scale + bottom
         end if
 
@@ -246,19 +247,20 @@ subroutine safe_coordinate_transform(x, y, x_min, x_max, y_min, y_max, &
         real(wp), parameter :: EPSILON = 1.0e-10_wp
         real(wp) :: x_range, y_range
         real(wp) :: x_scale, y_scale
-        
+
         ! Calculate ranges with epsilon protection
         x_range = x_max - x_min
         y_range = y_max - y_min
-        
-        ! Handle degenerate cases by centering
+
+        ! Map X using independent scale (center if degenerate)
         if (abs(x_range) < EPSILON) then
             pdf_x = plot_left + plot_width * 0.5_wp
         else
             x_scale = plot_width / x_range
             pdf_x = (x - x_min) * x_scale + plot_left
         end if
-        
+
+        ! Map Y using independent scale (center if degenerate)
         if (abs(y_range) < EPSILON) then
             pdf_y = plot_bottom + plot_height * 0.5_wp
         else