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/save_test.go

@@ -15,7 +15,7 @@ func TestSaveRestoreRoundTrip(t *testing.T) {
 	g.HasAmulet = true
 	g.Items.Potions[PotionHealing].Know = true
 	g.Items.Scrolls[ScrollMagicMapping].Guess = "map???"
-	g.Monsters['F'-'A'].Stats.Dmg = "3x1" // mutated bestiary must survive
+	g.Monsters['F'-'A'].Stats.Dmg = dice("3x1") // mutated bestiary must survive
 	if len(g.Level.Monsters) > 0 {
 		g.Level.Monsters[0].Flags.Set(Awake)
 		g.Level.Monsters[0].Dest = &g.Player.Pos
@@ -48,7 +48,7 @@ func TestSaveRestoreRoundTrip(t *testing.T) {
 	if h.Items.Scrolls[ScrollMagicMapping].Guess != "map???" {
 		t.Error("scroll guess lost")
 	}
-	if h.Monsters['F'-'A'].Stats.Dmg != "3x1" {
+	if h.Monsters['F'-'A'].Stats.Dmg.String() != "3x1" {
 		t.Error("mutated bestiary lost")
 	}
 	if h.Rng.Seed != g.Rng.Seed {