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>

.gitignore

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+.venv/
+__pycache__/
+*.pyc
+.pytest_cache/