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/core/GameVersion.lua

@@ -4,9 +4,10 @@
 -- extracted cache lives, and the save-file suffix -- so the importer,
 -- cache mount, SaveData, title screen and palette all agree.
 --
--- Red keeps every un-suffixed path it always used (save.lua, the root cache),
--- so existing installs are untouched; Blue is namespaced under blue/ and
--- _blue, Yellow under yellow/ and _yellow, so all three can be imported and
+-- Red keeps the un-suffixed save paths it always used (save.lua) so existing
+-- saves are untouched, but its extracted cache lives under red/ like Blue and
+-- Yellow (issue #899); a legacy root cache is moved into red/ once by
+-- CacheFs.migrateLegacyRedCache.  All three versions can be imported and
 -- played side by side.
 --
 -- Zero requires, so it loads during love.conf and under plain Lua for tools
@@ -23,7 +24,7 @@ GameVersion.VERSIONS = {
     launcherName = "Red",       -- game-panel header in the launcher
     sha1 = "ea9bcae617fdf159b045185467ae58b2e4a48b9a",
     manifest = "tools/rom_manifest.json",
-    cachePrefix = "",       -- Red owns the cache root (backwards compatible)
+    cachePrefix = "red/",   -- red/data/generated, red/assets/generated (#899)
     saveSuffix = "",        -- save.lua / save.lua.bak / save.lua.tmp
   },
   blue = {