feat(engine): SVG → Image asset loader (Card theme phase 1)

Implements the runtime SVG rasterisation pipeline that the card-theme
system (CARD_PLAN.md) is built on. Bevy 0.18 has no native SVG support;
this loader bridges usvg (parser) + resvg (renderer) + tiny-skia (CPU
pixmap) so the rest of the engine consumes themes as plain
Handle<Image>. Rasterisation happens once per (asset, settings) pair at
load time — Bevy's asset cache absorbs the cost.

solitaire_engine/src/assets/
  mod.rs           — module entrypoint
  svg_loader.rs    — SvgLoader (AssetLoader for .svg → Image)
                     SvgLoaderSettings { target_size: UVec2 } default 512×768
                     SvgLoaderError (Io / Parse / PixmapAlloc) via thiserror
                     rasterize_svg() helper exposed for non-asset-graph
                     callers (the future zip-importer validation step)

The rasteriser scales-to-fit while preserving aspect ratio, centring
the SVG inside the target box so a non-2:3 source doesn't pin to the
top-left corner.

7 new unit tests — default + custom target size, zero-dimension reject,
malformed-input reject, RGBA byte-count, extension advertisement, and
a compile-time guard that SvgLoaderSettings still satisfies the
AssetLoader::Settings trait bounds.

Workspace deps added: usvg 0.47, resvg 0.47, tiny-skia 0.12 (latest
minor versions; CARD_PLAN.md called out the placeholder numbers
needed verification).

cargo build / cargo clippy --workspace --all-targets -- -D warnings
/ cargo test --workspace all green (913 passed, 0 failed, 9 ignored —
+7 from the new loader tests).

solitaire_engine/Cargo.toml

@@ -13,6 +13,11 @@ solitaire_sync = { workspace = true }
 chrono         = { workspace = true }
 uuid           = { workspace = true }
 tokio          = { workspace = true }
+serde          = { workspace = true }
+thiserror      = { workspace = true }
+usvg           = { workspace = true }
+resvg          = { workspace = true }
+tiny-skia      = { workspace = true }
 
 [dev-dependencies]
 async-trait = { workspace = true }