Un-overload Object fields and pre-parse damage dice (refactor step 3)

Object.Arm carried four meanings in C (o_arm/o_charges/o_goldval plus
ring bonuses); it is now four fields: ArmorClass, Charges, GoldValue,
and Bonus. Stats.Arm becomes Stats.ArmorClass. The Charges()/GoldVal()
accessor pair is gone.

Damage dice strings ("1x4/1x2") are parsed once into DiceSpec — at
table definition for the bestiary and weapon tables, at creation for
items — instead of re-parsed with atoi on every swing as fight.c
roll_em did. ParseDice preserves the C parse semantics exactly,
including the junk-tolerant "%%%x0" bestiary placeholder and the
"000x0" flytrap reset (regression-tested in dice_test.go); the
flytrap's growing grip becomes DiceSpec{{VfHit, 1}}.

Save format bumps to 5.4.4-go3 (field renames and retypes would
silently zero under gob's match-by-name decoding).

No behavior change; full suite green.

game/fight_test.go

@@ -23,8 +23,8 @@ func spawnAdjacent(g *RogueGame, typ byte) *Monster {
 
 func TestRollEmParsesMultiAttackDice(t *testing.T) {
 	g := mkGame(t, 42)
-	att := &Creature{Stats: Stats{Str: 16, Lvl: 20, Dmg: "1x4/1x4/1x4"}}
-	def := &Creature{Stats: Stats{Arm: 10, HP: 1000}}
+	att := &Creature{Stats: Stats{Str: 16, Lvl: 20, Dmg: dice("1x4/1x4/1x4")}}
+	def := &Creature{Stats: Stats{ArmorClass: 10, HP: 1000}}
 	def.Flags.Set(Awake)
 	// With attacker level 20 vs armor 10, swing always hits
 	// (rnd(20)+wplus >= (20-20)-10 is always true), so three attacks of