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/newlevel.go

@@ -111,9 +111,9 @@ func (g *RogueGame) putThings() {
 	if g.Depth >= AmuletLevel && !g.HasAmulet {
 		obj := newObject()
 		attachObj(&g.Level.Objects, obj)
-		obj.Damage = "0x0"
-		obj.HurlDmg = "0x0"
-		obj.Arm = 11
+		obj.Damage = dice("0x0")
+		obj.HurlDmg = dice("0x0")
+		obj.ArmorClass = 11
 		obj.Kind = KindAmulet
 		// Put it somewhere
 		obj.Pos, _ = g.findFloor(nil, 0, false)