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/rooms.go

@@ -88,7 +88,7 @@ func (g *RogueGame) doRooms() {
 		if g.rnd(2) == 0 && (!g.HasAmulet || g.Depth >= g.MaxDepth) {
 			gold := newObject()
 			rp.GoldVal = g.goldCalc()
-			gold.SetGoldVal(rp.GoldVal)
+			gold.GoldValue = rp.GoldVal
 			rp.Gold, _ = g.findFloorIn(rp, 0, false)
 			gold.Pos = rp.Gold
 			g.Level.SetChar(rp.Gold.Y, rp.Gold.X, Gold)