platformer: fix the card backdrop seam (one cover-fill panel, not two stretches)

The illustrated-card backdrop stitched two copies of the same biome frame
stretched to fixed widths (0-592 and 432-1024); in their overlap each showed
a different part of the frame, so they did not line up.

Replace them with ONE panel scaled to COVER the card width with its aspect
kept (the overflow below the 640 card height is cropped by the window, and the
card's ground band hides the cropped foreground). A single image has no join,
so a seam is impossible - and the uniform scale means no distortion either.

Example-side only. Static gates + audit clean. Needs an OXT pass to confirm.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HkN9P6YodA65ZuhDDuuQks

Author: claude

examples/box2dxt-platformer.livecodescript

@@ -955,32 +955,36 @@ end pfCardCastMember
 -- blendLevel so the fade ramp can add progress on top without flattening the
 -- scrim/ground translucency.
 command pfCardArtBuildLevel
-   local tBg, tGY
+   local tBg, tGY, tBW, tBH, tBgH
    if gAssetsOK is not true then exit pfCardArtBuildLevel   -- placeholder mode: tint card only
    put 480 into tGY                                         -- the ground line the cast stands on
-   -- the biome backdrop (L1-L5), as two overlapping panels filling the 1024 width
+   -- the biome backdrop (L1-L5): ONE panel scaled to COVER the 1024 card width with
+   -- its aspect KEPT (the overflow below the 640 card height is simply cropped by
+   -- the window). A single image has no join, so there is no seam to misalign - the
+   -- earlier two-panel stretch mismatched in its overlap. The card's ground band
+   -- hides the cropped-off foreground.
    put pfLevelBgFrame(gLevel) into tBg
    if tBg is not empty and b2kSheetHasFrame("bg", tBg) then
       if pfCardArtImage("bg", tBg, "pfCardBgA") is not empty then
-         set the rect of image "pfCardBgA" to 0, 0, 592, 640
+         put the width of image "pfCardBgA" into tBW    -- natural sliced size (~640x640 at bg scale 2.5)
+         put the height of image "pfCardBgA" into tBH
+         if tBW < 1 then put 640 into tBW
+         if tBH < 1 then put 640 into tBH
+         put max(640, round(1024 * tBH / tBW)) into tBgH   -- aspect-scaled to fill the width (>= card height)
+         set the rect of image "pfCardBgA" to 0, 0, 1024, tBgH
          set the uCardBaseBlend of image "pfCardBgA" to 0
          pfCardAddArt the long id of image "pfCardBgA"
+         -- a dark scrim so the title text reads over the busy backdrop (rests at 42)
+         create graphic "pfCardScrim"
+         set the style of graphic "pfCardScrim" to "rectangle"
+         set the rect of graphic "pfCardScrim" to 0, 0, 1024, 640
+         set the filled of graphic "pfCardScrim" to true
+         set the backgroundColor of graphic "pfCardScrim" to "14,16,26"
+         set the blendLevel of graphic "pfCardScrim" to 42
+         set the uCardBaseBlend of graphic "pfCardScrim" to 42
+         set the visible of graphic "pfCardScrim" to false   -- hidden until pfCardShow reveals the whole cover at once
+         pfCardAddArt the long id of graphic "pfCardScrim"
       end if
-      if pfCardArtImage("bg", tBg, "pfCardBgB") is not empty then
-         set the rect of image "pfCardBgB" to 432, 0, 1024, 640
-         set the uCardBaseBlend of image "pfCardBgB" to 0
-         pfCardAddArt the long id of image "pfCardBgB"
-      end if
-      -- a dark scrim so the title text reads over the busy backdrop (rests at 42)
-      create graphic "pfCardScrim"
-      set the style of graphic "pfCardScrim" to "rectangle"
-      set the rect of graphic "pfCardScrim" to 0, 0, 1024, 640
-      set the filled of graphic "pfCardScrim" to true
-      set the backgroundColor of graphic "pfCardScrim" to "14,16,26"
-      set the blendLevel of graphic "pfCardScrim" to 42
-      set the uCardBaseBlend of graphic "pfCardScrim" to 42
-      set the visible of graphic "pfCardScrim" to false   -- hidden until pfCardShow reveals the whole cover at once
-      pfCardAddArt the long id of graphic "pfCardScrim"
    end if
    -- a ground band the cast stands on (a darker shade of the biome tint)
    create graphic "pfCardGround"