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/Sound.lua

@@ -269,13 +269,6 @@ function Sound.playPikaCry(data, n)
   if src == false then return nil end
   if not src then
     local path = ("assets/generated/audio/pika_cries/cry_%02d.wav"):format(n)
-    -- NX-only: Blue/Yellow live under a versioned save-dir prefix.
-    if require("src.core.Platform").isNX() then
-      local prefix = require("src.core.GameVersion").cachePrefix()
-      if prefix ~= "" and love.filesystem.getInfo(prefix .. path) then
-        path = prefix .. path
-      end
-    end
     local ok, s = pcall(love.audio.newSource, path, "static")
     if not ok or not s then
       cache[key] = false