fix(engine): wire FiraMono to Android corner label and add CardImageSet tests

Bug #1 (QS wrong watermark): extracted card_face_asset_path() pure helper so
the (Rank, Suit) → filename mapping is tested in isolation. 6 new unit tests
confirm all 52 keys are unique and each suit resolves to its correct letter.
QS.png has the wrong artwork baked in (confirmed via MD5); QS_BUG.md documents
the required asset replacement.

Bug #2/#3 (red square / invisible black suit on Android): add_android_corner_label
used TextFont { ..default() } which gives Bevy's built-in font — that font
lacks U+2660–U+2666, so suit glyphs rendered as a colored missing-glyph
rectangle. Threaded Option<&Handle<Font>> from sync_cards_startup/on_change →
sync_cards → spawn/update_card_entity → add_android_corner_label, which now
passes FiraMono explicitly. Non-Android builds silence the unused param with
let _ = font_handle.

Bug #4 (waste pile): static analysis found no z or fan-offset bug; two new
tests (waste_pile_cards_have_strictly_increasing_z, _draw_one_cards_have_distinct_z)
pin the invariant for future changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/cards/faces/classic/QS_BUG.md

@@ -0,0 +1,16 @@
+# QS.png — Asset Content Bug
+
+**Status:** Needs manual fix — replace `QS.png` with correct artwork.
+
+**Symptom:** The Queen of Spades card renders with a diamond watermark baked
+into the PNG artwork, while the top-left Android overlay correctly shows "Q♠".
+
+**Diagnosis:**
+- The code-side mapping (`card_face_asset_path(Rank::Queen, Suit::Spades)`)
+  correctly returns `"cards/faces/classic/QS.png"` — confirmed by unit test.
+- `QS.png` and `QD.png` have distinct MD5 hashes, so they are not the same
+  file. The bug is in the pixel content of `QS.png` itself.
+
+**Fix:** Replace `QS.png` with a correctly-drawn Queen of Spades image (spade
+watermark, not diamond). The image should be 120×168 px to match every other
+card face in this directory.