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

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

tests/engine/music_volume_hook_state.lua

@@ -0,0 +1,65 @@
+-- Regression: the `music.volume` mod hook must not crash on Music's private
+-- `state`.  applyVolume (src/core/Music.lua) builds its hook context from
+-- state.current/mapSong/onBike/... but was defined above the `local state`
+-- table, so those reads bound to the nil global `state` instead -- a mod that
+-- registered music.volume hit "attempt to index a nil value (global 'state')"
+-- the first time any volume was applied.  This drives a file-backed song
+-- through Music with the hook installed and asserts the context resolves.
+-- ROM-free: a fake audio source, no data/generated/.
+--   luajit tests/engine/music_volume_hook_state.lua
+
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local T = require("tests.harness")
+love = require("tests.love_stub")
+
+-- minimal audio source: only the methods Music calls on a file song
+local Source = {}
+Source.__index = Source
+function Source:play() self.playing = true end
+function Source:stop() self.playing = false end
+function Source:pause() self.playing = false end
+function Source:isPlaying() return self.playing end
+function Source:setLooping(v) self.looping = v end
+function Source:setVolume(v) self.volume = v end
+function Source:setFilter() end
+love.audio = {
+  newSource = function(file) return setmetatable({ file = file }, Source) end,
+}
+
+local Runtime = require("src.mods.Runtime")
+local Music = require("src.core.Music")
+
+local events = require("src.mods.Events").new()
+local hooks = require("src.mods.Hooks").new()
+Runtime.install(events, hooks)
+
+-- one file-backed song is enough; the chip path would pull in `bit`
+local data = { audio = { songs = { TEST = { file = "test.ogg" } } } }
+
+-- record every context the hook is handed, and scale the volume so the
+-- return value is exercised too
+local calls = {}
+hooks:wrap("music.volume", function(_next, vol, ctx)
+  calls[#calls + 1] = ctx
+  return vol * 0.5
+end, nil, "voltest")
+
+T.check(Runtime.wantsHook("music.volume"), "the music.volume hook is registered")
+
+-- Before the fix this call raised inside applyVolume; reaching the next line
+-- at all is the core of the regression.
+Music.play(data, "TEST")
+T.check(#calls >= 1, "applyVolume ran the hook during play without crashing")
+
+-- A second application, now that the song is current: state must resolve to
+-- the real playback table, so the context carries the live song + scale.
+local before = #calls
+Music.setVolumeLevel(5)
+T.check(#calls > before, "setVolumeLevel re-applies volume through the hook")
+local ctx = calls[#calls]
+T.check(type(ctx) == "table", "the hook receives a context table")
+T.eq(ctx.song, "TEST", "ctx.song is the live song (state resolved, not nil)")
+T.eq(ctx.optionScale, 5 / 7, "ctx.optionScale reflects the 0-7 volume level")
+
+T.finish("music_volume_hook_state")