Fix two latent bugs surfaced by static analysis

Both are code paths that never run in a green test today but crash or
misbehave the moment a mod or a link failure exercises them.

1. Music.lua: applyVolume built its `music.volume` hook context from the
   private `state` table, but was defined *above* `local state = {...}`, so
   those reads bound to the nil global `state`. Any mod registering the
   music.volume hook crashed with "attempt to index a nil value (global
   'state')" the first time a volume was applied. Forward-declare `state`
   above applyVolume. Regression test drives a file-backed song through the
   hook and asserts the context resolves.

2. Tournament.lua: `local battle, why = isHost and newHost() or newGuest()`
   had two defects. The and/or idiom truncates a call to its first result,
   so `why` (the specific failure reason) was always dropped and every link
   failure showed the generic "Link battle can't start" instead of e.g.
   "same mods on both games". Worse, when a host's newHost() returned nil,
   the `or` fell through and wrongly called newGuest() as the host. Split
   into an explicit if/else so the reason is preserved and each role calls
   its own constructor.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q6bFAiQyZ5jDmewsbB4LG9

src/link/Tournament.lua

@@ -384,9 +384,16 @@ function Tournament:update(dt)
         else
           self.pendingBattleOpts.seed = msg.seed
         end
-        local battle, why = self.isHost
-          and LinkBattle.newHost(self.game, self.net, self.pendingBattleOpts)
-          or LinkBattle.newGuest(self.game, self.net, self.pendingBattleOpts)
+        -- Split rather than `cond and newHost() or newGuest()`: the and/or
+        -- idiom truncates a call to its first result, so the second return
+        -- (the specific reason) was always dropped and every failure showed
+        -- the generic fallback instead of "same mods on both games" etc.
+        local battle, why
+        if self.isHost then
+          battle, why = LinkBattle.newHost(self.game, self.net, self.pendingBattleOpts)
+        else
+          battle, why = LinkBattle.newGuest(self.game, self.net, self.pendingBattleOpts)
+        end
         if not battle then
           self:exitWith(why or Strings("Link battle\ncan't start."))
           return