Add MTGA Assistant: native Linux deck and game tracker

PySide6 app that follows MTG Arena's Player.log under Steam/Proton and shows
my remaining library with draw odds, my zones, and everything revealed about
the opponent (board, graveyard, exile, known hand/library, seen this match).

- Auto-detects the Steam library, Proton prefix log and MTGA install
- Card names/costs/types from MTGA's local SQLite card database
- Tracks card identity across zone changes, shuffles and Bo3 games
- Scryfall card images with a lazy, size-capped LRU disk cache
- Live log following with restart detection, plus --replay mode
- Flags log gaps where MTGA omits oversized game-state messages

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_carddb.py

@@ -0,0 +1,50 @@
+import pytest
+
+from mtga_assistant import paths
+from mtga_assistant.carddb import CardDB, mana_value, parse_mana
+
+
+@pytest.mark.parametrize(
+    "text,symbols,mv",
+    [
+        ("o2oU", ["2", "U"], 3),
+        ("oW", ["W"], 1),
+        ("", [], 0),
+        ("o1o(R/G)", ["1", "R/G"], 2),
+        ("o1o(B/P)o(B/P)", ["1", "B/P", "B/P"], 3),
+        ("o(2/B)o(2/R)", ["2/B", "2/R"], 4),
+        ("oXoXoR", ["X", "X", "R"], 1),
+        ("o10", ["10"], 10),
+    ],
+)
+def test_parse_mana(text, symbols, mv):
+    assert parse_mana(text) == symbols
+    assert mana_value(symbols) == mv
+
+
+def test_missing_db_gives_placeholder():
+    db = CardDB(None)
+    assert not db.available
+    assert db.get(123).name == "Unknown card #123"
+    assert db.base_grp(123) == 123
+
+
+@pytest.fixture(scope="module")
+def real_db():
+    mtga = paths.find_mtga_dir(paths.load_config())
+    path = paths.find_card_db(mtga) if mtga else None
+    if path is None:
+        pytest.skip("MTGA is not installed on this machine")
+    return CardDB(path)
+
+
+def test_real_db_lookup(real_db):
+    island = real_db.get(97169)
+    assert island.name == "Island"
+    assert island.is_land
+
+
+def test_real_db_split_card_half_maps_to_whole(real_db):
+    # Desist (half) -> Cease // Desist
+    assert real_db.base_grp(89160) == 89158
+    assert real_db.base_grp(89158) == 89158