refactor(loaders): unify combatant loader, migrate PC features/spells

Rename monster.py → combatant.py with _KNOWN_FEATS lookup table
that resolves feat name strings to AbilitySpec constants (Power Attack,
Iron Will, Toughness, Weapon Focus, etc.). Unknown feats produce
minimal AbilitySpec (data preserved for future implementation).

Migrate all 8 PC JSONs (data/pcs/) with complete feat/spell lists
extracted from Foundry sheets — including traits, racial abilities,
and all spells. Monsters (goblin, orc) unchanged (no feats/spells).

Update all imports, add backward-compatible aliases (load_monster,
MonsterLoadError), update CLI to use load_combatant.

Gate: 377 tests, ruff clean, basedpyright clean.

data/pcs/ierlieth.json

@@ -62,5 +62,11 @@
       "count": 1
     }
   ],
-  "source": "fiches_personnages/ierlieth_sheet.json"
+  "source": "fiches_personnages/ierlieth_sheet.json",
+  "features": [
+    "Toughness",
+    "Power Attack",
+    "Drake (race)"
+  ],
+  "spells": []
 }