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/monsters.go

@@ -54,7 +54,7 @@ func (g *RogueGame) newMonster(tp *Monster, typ byte, cp Coord) {
 	tp.Stats.Lvl = mp.Stats.Lvl + levAdd
 	tp.Stats.MaxHP = g.roll(tp.Stats.Lvl, 8)
 	tp.Stats.HP = tp.Stats.MaxHP
-	tp.Stats.Arm = mp.Stats.Arm - levAdd
+	tp.Stats.ArmorClass = mp.Stats.ArmorClass - levAdd
 	tp.Stats.Dmg = mp.Stats.Dmg
 	tp.Stats.Str = mp.Stats.Str
 	tp.Stats.Exp = mp.Stats.Exp + levAdd*10 + expAdd(tp)
@@ -183,10 +183,10 @@ func (g *RogueGame) save(which int) bool {
 	p := &g.Player
 	if which == VsMagic {
 		if p.IsRing(Left, RingProtection) {
-			which -= p.CurRing[Left].Arm
+			which -= p.CurRing[Left].Bonus
 		}
 		if p.IsRing(Right, RingProtection) {
-			which -= p.CurRing[Right].Arm
+			which -= p.CurRing[Right].Bonus
 		}
 	}
 	return g.saveThrow(which, &p.Stats)