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/init.go

@@ -20,7 +20,7 @@ func (g *RogueGame) initPlayer() {
 	obj = newObject()
 	obj.Kind = KindArmor
 	obj.Which = int(ArmorRingMail)
-	obj.Arm = aClass[ArmorRingMail] - 1
+	obj.ArmorClass = aClass[ArmorRingMail] - 1
 	obj.Flags.Set(Known)
 	obj.Count = 1
 	p.CurArmor = obj