test(engine): pin card-face SVG output against rasteriser drift

Step 3 of the migration plan in docs/ui-mockups/card-face-migration.md.

Extracts face_svg / back_svg + palette constants from the
card_face_generator example into a new
solitaire_engine::assets::card_face_svg module so an integration
test can call them. The example becomes a thin wrapper.

The new tests/card_face_svg_pin.rs hashes the raw RGBA8 pixel
bytes from rasterising every face × suit + every back accent and
compares each FNV-1a fingerprint against an embedded constant.
Catches silent rendering drift if usvg / resvg / tiny_skia / the
bundled FiraMono ever change in a way that perturbs pixels.

Hashing is FNV-1a inline (~5 lines) rather than adding sha2 or
blake3 — cryptographic strength isn't load-bearing here, just
stable byte fingerprints.

When the SVG builders intentionally change, empty EXPECTED to
`&[]` and re-run the test once; it panics with the new hashes
formatted as Rust source ready to paste back in.

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

solitaire_engine/src/assets/mod.rs

@@ -6,6 +6,7 @@
 //! (user-themes directory). Phase 3 will extend it further with custom
 //! `AssetSource` implementations for `embedded://` and `themes://`.
 
+pub mod card_face_svg;
 pub mod sources;
 pub mod svg_loader;
 pub mod user_dir;