Move Red's extracted cache under red/ with a legacy migration

Importing Red unpacked data/generated, assets/generated and
rom-cache.complete straight into the save-dir root, while Blue and
Yellow land under blue/ and yellow/. Red now uses cachePrefix red/
like the others.

CacheFs.migrateLegacyRedCache moves a pre-existing root cache into
red/ on first boot, from RomImporter.new before the readiness loop
and from mountVersion, so existing installs keep their import instead
of being asked for the ROM again. The move only runs when the root
marker resolves to the save directory, so a dev checkout's source
tree is never touched, and the portable game folder is skipped when
it is the physfs source.

Closes #899

src/import/RomImporter.lua

@@ -32,7 +32,7 @@ end
 -- carry Red's bank $1f header, wave-table, and CryData offsets.
 local CACHE_FORMAT = "rom-cache-v9:"
 -- The completion marker is written under each version's cache prefix
--- (rom-cache.complete for Red, blue/rom-cache.complete for Blue).
+-- (red/rom-cache.complete, blue/rom-cache.complete, ...).
 local MARKER_PATH = "rom-cache.complete"
 
 -- The marker a finished import writes for a version: the generation tag plus
@@ -135,8 +135,8 @@ local PAL = {
 
 -- CacheFs.exists checks the game folder directly for a portable install,
 -- otherwise the save directory through love.filesystem.  It honors
--- CacheFs.prefix, so we point it at the version's cache subtree (Red at the
--- root, Blue under blue/).
+-- CacheFs.prefix, so we point it at the version's cache subtree (red/,
+-- blue/, yellow/).
 local function allRequiredFilesExist(version)
   local CacheFs = require("src.import.CacheFs")
   local saved = CacheFs.prefix
@@ -153,11 +153,15 @@ local function allRequiredFilesExist(version)
 end
 
 -- A developer checkout / Python build leaves Red's generated data in the
--- physfs SOURCE at the un-prefixed root; that is always current.  Only Red
--- ships this way (Blue is import-only), so this stays a Red-root check.
+-- physfs SOURCE at the un-prefixed root (the checked-out data/generated and
+-- assets/generated); it is always current and never moves into red/.  Only
+-- Red ships this way (Blue/Yellow are import-only).  The check goes through
+-- love.filesystem directly so the red/ cache prefix cannot hide the source
+-- tree, and the realDirectory test keeps a save-dir cache from counting.
 local function sourceTreeHasData()
-  if not allRequiredFilesExist("red") or not love.filesystem.getRealDirectory then
-    return false
+  if not love.filesystem.getRealDirectory then return false end
+  for _, path in ipairs(REQUIRED_FILES) do
+    if love.filesystem.getInfo(path, "file") == nil then return false end
   end
   local real = love.filesystem.getRealDirectory(REQUIRED_FILES[1])
   return real == love.filesystem.getSource()
@@ -214,8 +218,8 @@ local function purgeSaveDirCache()
     f:close()
     return true
   end
-  -- Purge each version's stale save-directory copy (Red at the root, Blue
-  -- under blue/) so it cannot shadow the portable game-folder cache.
+  -- Purge each version's stale save-directory copy (under its red/ / blue/
+  -- / yellow/ prefix) so it cannot shadow the portable game-folder cache.
   for _, version in ipairs(GameVersion.ORDER) do
     local prefix = GameVersion.cachePrefix(version)
     if saveDirHas(prefix .. MARKER_PATH) or saveDirHas(prefix .. REQUIRED_FILES[1]) then
@@ -1121,6 +1125,11 @@ function RomImporter.new(onComplete, opts)
     _padInited = false,
   }, RomImporter)
 
+  -- Pre-#899 installs keep Red's extracted cache at the save-dir root; move
+  -- it under red/ before the readiness loop looks for red/ paths, or every
+  -- such install would read as "never imported" and demand the ROM again.
+  CacheFs.migrateLegacyRedCache()
+
   for _, version in ipairs(GameVersion.ORDER) do
     local info = GameVersion.info(version)
     local ready = RomImporter.isReady(version) and not self.forceImport