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/object.go

@@ -94,21 +94,24 @@ func (k ObjectKind) MergesInPack() bool {
 // Object is the _o arm of the C THING union: anything that can lie on the
 // floor or ride in a pack.
 type Object struct {
-	Kind    ObjectKind // what kind of object it is (o_type)
-	Pos     Coord      // where it lives on the screen
-	Text    string     // what it says if you read it
-	Launch  WeaponKind // what you need to launch it (noWeapon if none)
-	PackCh  byte       // what character it is in the pack
-	Damage  string     // damage if used like sword
-	HurlDmg string     // damage if thrown
-	Count   int        // count for plural objects
-	Which   int        // which object of a type it is (index for the Kind's table)
-	HPlus   int        // plusses to hit
-	DPlus   int        // plusses to damage
-	Arm     int        // armor protection — overloaded as in C (see Charges/GoldVal)
-	Flags   ObjFlags
-	Group   int    // group number for this object
-	Label   string // label for object
+	Kind       ObjectKind // what kind of object it is (o_type)
+	Pos        Coord      // where it lives on the screen
+	Text       string     // what it says if you read it
+	Launch     WeaponKind // what you need to launch it (noWeapon if none)
+	PackCh     byte       // what character it is in the pack
+	Damage     DiceSpec   // damage if used like sword
+	HurlDmg    DiceSpec   // damage if thrown
+	Count      int        // count for plural objects
+	Which      int        // which object of a type it is (index for the Kind's table)
+	HPlus      int        // plusses to hit
+	DPlus      int        // plusses to damage
+	ArmorClass int        // armor protection (armor only)
+	Charges    int        // charges remaining (wands and staffs only)
+	GoldValue  int        // worth (gold piles only)
+	Bonus      int        // magic bonus (rings only: protection, add strength, ...)
+	Flags      ObjFlags
+	Group      int    // group number for this object
+	Label      string // label for object
 }
 
 // newObject is list.c new_item() for objects: a zeroed Object with the
@@ -117,18 +120,6 @@ func newObject() *Object {
 	return &Object{Launch: noWeapon}
 }
 
-// Charges is the o_charges alias for Arm on wands and staffs.
-func (o *Object) Charges() int { return o.Arm }
-
-// SetCharges stores a charge count in the overloaded Arm field.
-func (o *Object) SetCharges(n int) { o.Arm = n }
-
-// GoldVal is the o_goldval alias for Arm on gold piles.
-func (o *Object) GoldVal() int { return o.Arm }
-
-// SetGoldVal stores a gold value in the overloaded Arm field.
-func (o *Object) SetGoldVal(n int) { o.Arm = n }
-
 // PotionKind returns Which as a potion kind; valid only for KindPotion.
 func (o *Object) PotionKind() PotionKind { return PotionKind(o.Which) }