go: port dungeon generation, items base, pack, and monster creation

rooms.c, passages.c, new_level.c ported in full: room/maze layout,
corridor spanning tree with extra connections, passage numbering flood
fill, trap/stairs/object placement, treasure rooms. Careful RNG-call
ordering throughout keeps generation seed-faithful to C.

Supporting subsystems this depends on, also ported:
- screen.go: curses replaced by in-memory Window cell buffers behind a
  Terminal interface (tcell arrives with the UI phase; tests run headless)
- io.go: msg/addmsg/endmsg message-line protocol, status line, step_ok
- pack.c in full (add_pack sorting/stacking walk, get_item, inventory)
- things.c in full (inv_name, new_thing, discovery lists, pagination)
- monsters.c in full; chase.c navigation half (roomin/cansee/see_monst/
  find_dest/runto/set_oldch); rings.c, armor.c in full
- weapons.c/sticks.c creation half (init_weapon, fix_stick, num)
- misc.c look() display update, eat, level-up, direction input
- daemons.c callbacks except stomach (needs death()) and the runners
  chase driver (combat phase)
- init.c init_player with the starting kit

Tests: level invariants across seeds, determinism, 30-depth sweep.

game/potions.go

@@ -0,0 +1,85 @@
+package game
+
+// potions.c — the visibility-related helpers arrive first; quaff and the
+// potion effect dispatch come with the effects phase.
+
+// isMagic reports whether an object radiates magic (potions.c is_magic).
+func (o *Object) isMagic() bool {
+	switch o.Type {
+	case Armor:
+		return o.Flags.Has(IsProt) || o.Arm != aClass[o.Which]
+	case Weapon:
+		return o.HPlus != 0 || o.DPlus != 0
+	case Potion, Scroll, Stick, Ring, Amulet:
+		return true
+	}
+	return false
+}
+
+// invisOn turns on the ability to see invisible monsters (potions.c
+// invis_on).
+func (g *RogueGame) invisOn() {
+	g.Player.Flags.Set(CanSee)
+	for _, mp := range g.Level.Monsters {
+		if mp.On(IsInvis) && g.seeMonst(mp) && !g.Player.On(IsHalu) {
+			g.mvaddch(mp.Pos.Y, mp.Pos.X, mp.Disguise)
+		}
+	}
+}
+
+// turnSee puts on or off seeing monsters on this level (potions.c
+// turn_see).
+func (g *RogueGame) turnSee(turnOff bool) bool {
+	addNew := false
+	for _, mp := range g.Level.Monsters {
+		g.move(mp.Pos.Y, mp.Pos.X)
+		canSee := g.seeMonst(mp)
+		if turnOff {
+			if !canSee {
+				g.addch(mp.OldCh)
+			}
+		} else {
+			if !canSee {
+				g.standout()
+			}
+			if !g.Player.On(IsHalu) {
+				g.addch(mp.Type)
+			} else {
+				g.addch(byte(g.rnd(26) + 'A'))
+			}
+			if !canSee {
+				g.standend()
+				addNew = true
+			}
+		}
+	}
+	if turnOff {
+		g.Player.Flags.Clear(SeeMonst)
+	} else {
+		g.Player.Flags.Set(SeeMonst)
+	}
+	return addNew
+}
+
+// seenStairs reports whether the player has seen the stairs (potions.c
+// seen_stairs).
+func (g *RogueGame) seenStairs() bool {
+	st := g.Level.Stairs
+	g.move(st.Y, st.X)
+	if g.inch() == Stairs { // it's on the map
+		return true
+	}
+	if g.Player.Pos == st { // it's under him
+		return true
+	}
+	// if a monster is on the stairs, this gets hairy
+	if tp := g.Level.MonsterAt(st.Y, st.X); tp != nil {
+		if g.seeMonst(tp) && tp.On(IsRun) { // visible and awake:
+			return true // it must have moved there
+		}
+		if g.Player.On(SeeMonst) && tp.OldCh == Stairs {
+			return true
+		}
+	}
+	return false
+}