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/io.go

@@ -142,9 +142,9 @@ func (g *RogueGame) status() {
 	p := &g.Player
 
 	// If nothing has changed since the last status, don't bother.
-	temp := p.Stats.Arm
+	temp := p.Stats.ArmorClass
 	if p.CurArmor != nil {
-		temp = p.CurArmor.Arm
+		temp = p.CurArmor.ArmorClass
 	}
 	if s.init && s.hp == p.Stats.HP && s.exp == p.Stats.Exp &&
 		s.pur == p.Purse && s.arm == temp && s.str == p.Stats.Str &&