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/misc.go

@@ -279,10 +279,10 @@ func (g *RogueGame) chgStr(amt int) {
 	addStr(&p.Stats.Str, amt)
 	comp := p.Stats.Str
 	if p.IsRing(Left, RingAddStrength) {
-		addStr(&comp, -p.CurRing[Left].Arm)
+		addStr(&comp, -p.CurRing[Left].Bonus)
 	}
 	if p.IsRing(Right, RingAddStrength) {
-		addStr(&comp, -p.CurRing[Right].Arm)
+		addStr(&comp, -p.CurRing[Right].Bonus)
 	}
 	if comp > p.MaxStats.Str {
 		p.MaxStats.Str = comp