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/core/Music.lua

@@ -23,6 +23,13 @@ local volumeScale = 1
 local FILTER_HIGHGAIN = { 0.4, 0.16, 0.064 }
 local filterLevel = 0
 
+-- Forward-declared here so applyVolume (below) closes over the real playback
+-- state rather than a nil global: the table literal is assigned further down,
+-- but a `local state = {}` there would leave every reference above it bound
+-- to the global `state`.  Before this, registering the `music.volume` mod
+-- hook crashed applyVolume on `state.current` (a nil index).
+local state
+
 local function applyVolume(src)
   if not src then return end
   local vol = VOLUME * volumeScale
@@ -63,7 +70,7 @@ local function applyFilter(src)
   end
 end
 
-local state = {
+state = {
   current = nil,      -- song label
   chip = false,       -- the playing song is a synthesized channel program
   source = nil,       -- currently playing source