fix(switch): centralize the NX asset fallback in a boot-time loader overlay

The scattered per-call-site prefix rewrites were a parallel track that any
future newImage("assets/generated/...") would silently bypass.  Replace
them with NxAssetOverlay: installed once from love.load on NX only, it
wraps newImage / newImageData / newSource / filesystem.read / getInfo so a
missing assets/generated path falls back to the active version's
blue|yellow copy.  Call sites return to plain love loader calls, and
Assets.resolve goes back to being the platform-free mod-override point.

Two deliberate exceptions remain: the chip-audio worker (separate Lua
state) keeps receiving the prefix explicitly via audio.programPrefix, and
data/generated module loads keep using CacheFs.readActive.

A new guard test (tests/engine/nx_generated_guard_test.lua) fails CI on
any direct love loader call with a literal assets/generated path, so the
class of bug cannot regress by accident.  scripts/test.sh --quick is
green across all tiers.

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

src/core/NxAssetOverlay.lua

@@ -0,0 +1,95 @@
+-- NX-only asset overlay: fused love-nx cannot reliably mount
+-- blue|yellow/assets/generated onto the un-prefixed assets/generated, so
+-- instead of teaching every call site about versioned caches, this module
+-- wraps the love loading entry points ONCE at boot: any string path under
+-- assets/generated/ that does not resolve falls back to the active
+-- version's prefixed copy (yellow|blue/assets/generated/...).
+--
+-- main.lua installs it only when Platform.isNX(); desktop/Android/iOS never
+-- install it, so their mountVersion overlay stays the single mechanism and
+-- their loaders keep stock behavior.  Writes are deliberately NOT wrapped:
+-- the importer must keep targeting the versioned tree explicitly.
+--
+-- Two intentional exceptions stay outside this module:
+--   * the chip-audio worker (src/core/chip_worker.lua) is a separate Lua
+--     state without these wrappers; ChipAudio.slimAudio hands it the prefix
+--     explicitly as audio.programPrefix.
+--   * data/generated module loads go through CacheFs.readActive, which
+--     already implements the same fallback for require bytes.
+
+local GameVersion = require("src.core.GameVersion")
+
+local GENERATED = "assets/generated/"
+
+local NxAssetOverlay = {}
+
+local originals -- raw love functions, non-nil while installed
+
+-- Resolve `path` to the versioned copy when the un-prefixed file is missing
+-- and the active version (Blue/Yellow) carries it.  Returns nil when the
+-- caller's path should be used untouched (non-generated path, Red, the real
+-- file exists, or no versioned copy).
+local function versioned(path)
+  if type(path) ~= "string" then return nil end
+  if path:sub(1, #GENERATED) ~= GENERATED then return nil end
+  local prefix = GameVersion.cachePrefix()
+  if prefix == "" then return nil end
+  if originals.getInfo(path) then return nil end
+  local candidate = prefix .. path
+  if originals.getInfo(candidate) then return candidate end
+  return nil
+end
+
+local function wrapLoader(fn)
+  return function(path, ...)
+    local alt = versioned(path)
+    if alt then return fn(alt, ...) end
+    return fn(path, ...)
+  end
+end
+
+function NxAssetOverlay.isInstalled()
+  return originals ~= nil
+end
+
+function NxAssetOverlay.install()
+  if originals then return end
+  if not (love and love.filesystem) then return end
+  originals = {
+    read = love.filesystem.read,
+    getInfo = love.filesystem.getInfo,
+    newImage = love.graphics and love.graphics.newImage,
+    newImageData = love.image and love.image.newImageData,
+    newSource = love.audio and love.audio.newSource,
+  }
+  love.filesystem.read = wrapLoader(originals.read)
+  love.filesystem.getInfo = function(path, ...)
+    local alt = versioned(path)
+    if alt then return originals.getInfo(alt, ...) end
+    return originals.getInfo(path, ...)
+  end
+  if originals.newImage then
+    love.graphics.newImage = wrapLoader(originals.newImage)
+  end
+  if originals.newImageData then
+    love.image.newImageData = wrapLoader(originals.newImageData)
+  end
+  if originals.newSource then
+    love.audio.newSource = wrapLoader(originals.newSource)
+  end
+end
+
+-- Tests restore the stock loaders between cases; the game never uninstalls.
+function NxAssetOverlay.uninstall()
+  if not originals then return end
+  love.filesystem.read = originals.read
+  love.filesystem.getInfo = originals.getInfo
+  if originals.newImage then love.graphics.newImage = originals.newImage end
+  if originals.newImageData then
+    love.image.newImageData = originals.newImageData
+  end
+  if originals.newSource then love.audio.newSource = originals.newSource end
+  originals = nil
+end
+
+return NxAssetOverlay