fix(switch): prefer Yellow/Blue asset bytes when mount overlay lies

Probe generated canaries after mountVersion and always readActive for
prefixed caches so sprites are not blanked by empty PhysFS stubs.

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

src/import/CacheFs.lua

@@ -409,6 +409,32 @@ local function mountGeneratedTrees(prefix)
   return mounted
 end
 
+-- True when unprefixed generated cache is actually readable (not merely a
+-- directory stub PhysFS can see). Used after Blue/Yellow mounts so NX cannot
+-- silently boot with a broken overlay.
+local function generatedCacheVisible()
+  if not (love and love.filesystem) then return false end
+  local canaries = {
+    "assets/generated/fonts/font.png",
+    "data/generated/constants.lua",
+  }
+  for _, path in ipairs(canaries) do
+    local bytes = love.filesystem.read(path)
+    if type(bytes) == "string" and #bytes > 0 then return true end
+  end
+  return false
+end
+
+local function noteMountProbe(version, prefix, ok)
+  local okReq, Diag = pcall(require, "src.debug.SwitchDiagnostics")
+  if okReq and Diag and Diag.onEvent then
+    Diag.onEvent(ok and "mount_probe_ok" or "mount_probe_failed", {
+      version = tostring(version or ""),
+      prefix = tostring(prefix or ""),
+    })
+  end
+end
+
 function CacheFs.mountVersion(version)
   local prefix = require("src.core.GameVersion").cachePrefix(version)
   local sub = prefix:gsub("/+$", "")
@@ -432,6 +458,27 @@ function CacheFs.mountVersion(version)
 
   -- Version-scoped generated trees → un-prefixed paths (Red prefix is "").
   mountGeneratedTrees(prefix)
+
+  -- Blue/Yellow: verify the overlay actually exposes generated files. A failed
+  -- mount still returns true so Play can fall back through CacheFs.readActive,
+  -- but we retry once and leave a SwitchDiagnostics breadcrumb when enabled.
+  if sub ~= "" then
+    if not generatedCacheVisible() then
+      mountGeneratedTrees(prefix)
+    end
+    local ok = generatedCacheVisible()
+    noteMountProbe(version, prefix, ok)
+    if not ok then
+      -- Last resort: root mount alone sometimes leaves assets/ hidden behind
+      -- fused archive assets/; re-issue both mounts once more.
+      if love.filesystem.mount and love.filesystem.getInfo(sub, "directory") then
+        love.filesystem.mount(sub, "", false)
+      end
+      mountGeneratedTrees(prefix)
+      noteMountProbe(version, prefix, generatedCacheVisible())
+    end
+  end
+
   return true
 end