feat(engine): CardTheme asset + manifest loader (Card theme phase 2)

Implements Phase 2 of CARD_PLAN.md — the data types and `.theme.ron`
asset loader that build on Phase 1's SVG rasteriser.

solitaire_engine/src/theme/
  mod.rs        — CardKey { suit, rank } as the HashMap lookup key
                  (distinct from solitaire_core::Card which carries
                  per-deal id + face_up state); CardKey::all() yields
                  the 52 keys in suit-major / rank-ascending order;
                  manifest_name() and parse_manifest_name() round-trip
                  via the canonical "{suit}_{rank}" form.
                  ThemeMeta with structural validation (id non-empty,
                  no path separators, non-zero aspect components).
                  CardTheme #[derive(Asset, TypePath)] storing the
                  53 image handles + meta.
  manifest.rs   — ThemeManifest { meta, back, faces } with serde for
                  RON round-trip. validate() returns a strongly-typed
                  HashMap<CardKey, PathBuf>, surfacing precise errors
                  for unknown face keys, missing-of-52 entries, and
                  duplicate keys (RON silently keeps the last; brittle
                  for a release).
  loader.rs     — AssetLoader for .theme.ron. Validates manifest, then
                  composes sibling SVG paths via AssetPath::resolve so
                  the same loader works for both embedded:// and
                  themes:// asset sources (Phase 3 territory).
                  Schedules every face + back load through SvgLoader
                  with target_size derived from meta.card_aspect.

24 new tests covering: 52-key enumeration uniqueness, manifest-name
round trip, garbage-name rejection, complete/missing/unknown/duplicate
manifest validation, RON round-trip integrity, target-size aspect
math (2:3 → 512x768; non-standard; degenerate 1:10000 clamps to 1px).

Workspace deps added: ron 0.12.

cargo build / clippy --workspace --all-targets -- -D warnings / test
all green (937 passed total — +24 from Phase 2 vs the +7 from
Phase 1's b8fb3fb baseline).

Cargo.lock

@@ -7679,6 +7679,7 @@ dependencies = [
  "chrono",
  "kira",
  "resvg",
+ "ron",
  "serde",
  "solitaire_core",
  "solitaire_data",