fix(switch): extend NX-only asset prefix to audio and title/intro art

The previous NX gate only rewrote image paths that go through Assets.resolve.
Pokemon Yellow still had no sound and a blank title screen because:

- ChipSynth reads programs.bin directly via love.filesystem.read, bypassing
  Assets. On NX the unprefixed path is missing when the mount overlay fails,
  so the engine never built and every song/SFX was silent.
- Sound.playPikaCry loads pika_cries WAVs with love.audio.newSource, also
  bypassing Assets.resolve.
- TitleState, YellowIntro, and IntroMovie call love.graphics.newImage
  directly on unprefixed assets/generated paths, so the Pikachu title and
  intro atlases failed to load.

Fix: apply the same NX-only prefix rewrite in those four places.
Desktop/Android keep the existing mountVersion overlay behavior.

Also add ChipSynth._loadBanksForTest and tests covering the new paths.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/core/ChipSynth.lua

@@ -123,7 +123,18 @@ local function loadBanks(data)
   if cachedProgramFile == audio.programFile and cachedBanks then
     return cachedBanks
   end
-  local raw, readError = love.filesystem.read(audio.programFile)
+  local raw, readError
+  -- NX-only: Blue/Yellow live under a versioned save-dir prefix; desktop
+  -- relies on mountVersion overlay. Read the prefixed path when present.
+  if require("src.core.Platform").isNX() then
+    local prefix = require("src.core.GameVersion").cachePrefix()
+    if prefix ~= "" then
+      raw, readError = love.filesystem.read(prefix .. audio.programFile)
+    end
+  end
+  if not raw then
+    raw, readError = love.filesystem.read(audio.programFile)
+  end
   if not raw then error("could not read sound programs: " .. tostring(readError)) end
   local banks = {}
   for index, bank in ipairs(audio.bankOrder) do
@@ -140,6 +151,11 @@ function ChipSynth.invalidateBanks()
   cachedProgramFile, cachedBanks = nil, nil
 end
 
+-- test-only: exercise loadBanks without building a full engine
+function ChipSynth._loadBanksForTest(data)
+  return loadBanks(data)
+end
+
 -- A def-local program (ChipAsm output) is mounted as pseudo-bank 0 next to
 -- the ROM banks, so the 0x4000-window byte reader and every call/loop
 -- target work unchanged.  The ROM's own cached bank table is never touched