fix(engine): drop card-face border to remove gray-corner artifact

Player feedback after the 2-colour revert: "I do not like the
grey corners on the cards." The visible artifact was anti-
aliasing physics — the 1 px suit-coloured stroke (red for
hearts/diamonds, near-white for clubs/spades) faded through
gray pixels into the dark play surface at each rounded corner,
producing a visible "gray sliver" at the four arcs of every
card.

Fix: drop the stroke entirely. The card body fill defines the
shape against the play surface; the 5-unit brightness gap
between `#1a1a1a` body and `#151515` surface is enough to read
as a card edge without an explicit stroke. Anti-aliasing on a
fill-only rounded rect blends `#1a1a1a → #151515` over a few
pixels — barely perceptible compared to the
`stroke → transparent` gradient that produced the artifact.

### Changes

- `card_face_svg.rs`: removed `stroke="{colour}" stroke-width="2"`
  from the card body rect. Reverted the 1 px stroke inset back
  to `(x=0, y=0, width=256, height=384)` since there's no
  longer a stroke to keep inside the pixmap. Module-level
  comment updated to document the reasoning.
- `design-system.md` § Game Cards line 225 updated: "Border:
  1px solid in suit color" → "Border: none." with the
  artifact rationale recorded as audit trail.
- `card_face_svg_pin.rs` rebaselined: all 52 face hashes drift
  (every card's perimeter pixels changed); 5 back hashes
  unchanged.

Workspace clippy + cargo test --workspace clean. 1191 passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/assets/card_face_svg.rs

@@ -213,15 +213,21 @@ pub fn face_svg(rank: Rank, suit: Suit) -> String {
     let large_glyph_attrs = glyph_paint_attrs(colour, paint);
 
     // Numbers come from `design-system.md` § Game Cards, scaled 2×:
-    //   border:        1 px  → 2 px  stroke-width
     //   corner radius: 8 px  → 16 px rx/ry
     //   rank font:    18 px  → 36 px
     //   small glyph:  10 px  → 20 px (suit_path_d is authored at 32 →
     //                                 scale 0.625 to land at 20)
     //   large glyph:  32 px  → 64 px (scale 2.0)
     //
-    // Inset the border by 1 px so the 2 px stroke renders fully
-    // inside the 256 × 384 pixmap rather than getting clipped.
+    // No stroke on the card body. The earlier 1 px suit-coloured
+    // border created anti-aliasing artifacts at the rounded corners
+    // where the colored stroke faded through gray pixels into the
+    // dark play surface beneath — a visible "gray sliver" at each
+    // rounded corner that read as a rendering bug. Without the
+    // stroke, the card shape is defined purely by the body fill
+    // against the play surface; the 5-unit brightness gap between
+    // `#1a1a1a` (card body) and `#151515` (play surface) is small
+    // enough that anti-aliasing is barely perceptible.
     //
     // Suit glyphs are rendered as inline SVG paths (not `<text>`)
     // because the bundled `FiraMono` font doesn't carry usable
@@ -236,8 +242,8 @@ pub fn face_svg(rank: Rank, suit: Suit) -> String {
     // `design-system.md` § Game Cards for the spec deviation.
     format!(
         r##"<svg xmlns="http://www.w3.org/2000/svg" width="256" height="384" viewBox="0 0 256 384">
-  <rect x="1" y="1" width="254" height="382" rx="16" ry="16"
-        fill="{BG_FACE}" stroke="{colour}" stroke-width="2"/>
+  <rect x="0" y="0" width="256" height="384" rx="16" ry="16"
+        fill="{BG_FACE}"/>
 
   <!-- Top-left rank in JetBrains-Mono-styled FiraMono (rank digits
        and letters render correctly in FiraMono; only the suit glyphs