fix(engine): regenerate default theme SVGs to Terminal aesthetic

Step 4's PNG regeneration left the cards looking unchanged at
runtime because the PNGs at assets/cards/ are only the *fallback*
art — production renders the bundled-default theme's SVGs, which
get include_bytes!()-embedded into the binary by
solitaire_engine::assets::sources and applied to CardImageSet at
startup by theme::plugin::apply_theme_to_card_image_set. Those
SVGs were still the legacy vector-playing-cards art.

Extends card_face_generator to write SVGs into both runtime
paths in lockstep:

1. assets/cards/{faces,backs}/*.png — fallback art (unchanged
   from step 4).
2. solitaire_engine/assets/themes/default/*.svg — what production
   actually renders. 52 face SVGs + 1 back SVG, generated from
   the same face_svg / back_svg builders as the PNGs so the two
   paths can never visually diverge.

Adds two helper functions to card_face_svg:

- theme_suit_token (clubs/diamonds/hearts/spades — lowercase
  full word, matching CardKey::manifest_name)
- theme_rank_token (ace/2..10/jack/queen/king — same)

The theme back uses BACK_ACCENTS[0] (canonical Terminal cyan).
The other four accents only live as PNG fallbacks because the
theme system carries one back per theme.

Net SVG diff: -14884 / +940 lines — the legacy vector-playing-
cards SVGs were ~300 lines each of Inkscape-authored paths;
the Terminal SVGs are ~10 lines of programmatic output.

Workspace clippy + cargo test --workspace clean. Pin test
unaffected (the SVG builders themselves did not change).

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

solitaire_engine/src/assets/card_face_svg.rs

@@ -103,6 +103,39 @@ pub fn suit_filename(suit: Suit) -> &'static str {
     }
 }
 
+/// Lowercase full-word suit token used by the bundled-default theme's
+/// SVG filenames (`<suit>_<rank>.svg` — e.g. `spades_ace.svg`). Mirrors
+/// `solitaire_engine::theme::CardKey::manifest_name`.
+pub fn theme_suit_token(suit: Suit) -> &'static str {
+    match suit {
+        Suit::Clubs => "clubs",
+        Suit::Diamonds => "diamonds",
+        Suit::Hearts => "hearts",
+        Suit::Spades => "spades",
+    }
+}
+
+/// Lowercase full-word rank token used by the bundled-default theme's
+/// SVG filenames (`ace`, `2`..`10`, `jack`, `queen`, `king`). Mirrors
+/// `solitaire_engine::theme::CardKey::manifest_name`.
+pub fn theme_rank_token(rank: Rank) -> &'static str {
+    match rank {
+        Rank::Ace => "ace",
+        Rank::Two => "2",
+        Rank::Three => "3",
+        Rank::Four => "4",
+        Rank::Five => "5",
+        Rank::Six => "6",
+        Rank::Seven => "7",
+        Rank::Eight => "8",
+        Rank::Nine => "9",
+        Rank::Ten => "10",
+        Rank::Jack => "jack",
+        Rank::Queen => "queen",
+        Rank::King => "king",
+    }
+}
+
 #[derive(Copy, Clone)]
 enum GlyphPaint {
     Filled,