fix: serve australian mega cd discs their pal bios

BlastEm and Kega Fusion pick the European Mega CD BIOS for every disc
whose region code is the PAL one, Australian discs included: BlastEm
reads E or bit 8 of the header region (romdb.c:176-215), Fusion the
security byte 0x64 at 0x20B. Both now carry oceania, and a repo-wide
test holds every European Mega CD BIOS to it.

Fusion picks its export Master System BIOS from the CountryOrder
setting or a CRC32 list, never from the cart, so bios_U.sms and
bios_E.sms carry no region.

emulators/blastem.yml

@@ -80,10 +80,10 @@ files:
 
   - name: bios_CD_E.bin
     system: sega-megacd
-    region: [europe]
+    region: [europe, oceania]
     required: true
     mode: libretro
-    note: "Mega CD BIOS for discs whose region resolves to E (segacd.c:1587-1588). Same load rules as bios_CD_U.bin."
+    note: "Mega CD BIOS for discs whose region resolves to E (segacd.c:1587-1588). The header code E, or bit 8 of a hex code, is the only PAL value the code reads, so Australian discs take this branch too (romdb.c:176-215). Same load rules as bios_CD_U.bin."
     source_ref: "source libblastem.c:502, source segacd.c:1573-1615"
 
   - name: bios_CD_J.bin