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/types.go

@@ -374,13 +374,13 @@ type Coord struct {
 
 // Stats describes a fighting being (rogue.h struct stats).
 type Stats struct {
-	Str   int    // strength (s_str; 3..31)
-	Exp   int    // experience
-	Lvl   int    // level of mastery
-	Arm   int    // armor class
-	HP    int    // hit points (s_hpt)
-	Dmg   string // damage dice, e.g. "1x4/1x2"
-	MaxHP int
+	Str        int      // strength (s_str; 3..31)
+	Exp        int      // experience
+	Lvl        int      // level of mastery
+	ArmorClass int      // armor class (s_arm)
+	HP         int      // hit points (s_hpt)
+	Dmg        DiceSpec // damage dice, e.g. "1x4/1x2"
+	MaxHP      int
 }
 
 // Room describes a room or passage network (rogue.h struct room).