explore: direction arrows showing where you can move

Overlay real ▲▼◀▶ arrow sprites at the screen edges for every direction the
camera can currently page: either there's more map that way, or the map has a
connection that way. Arrows update live -- e.g. at a connectionless edge that
direction's arrow disappears.

Implementation: the font has only a down-arrow ($ee) and right-arrow ($ed), so
up/left are made with CGB OBJ Y/X-flip (the emulator supports OBJ flips). Drawn
as 4 sprites in wShadowOAM (8x8 OBJ mode, font loaded so $ed/$ee are OBJ-
reachable at 0x8ed0/0x8ee0). Set wUpdateSpritesEnabled=$ff after building them so
the vblank PrepareOAMData leaves shadow OAM alone instead of repopulating the
player sprite over the arrows.

Verified: Pallet mid-map shows all 4; Viridian shows N/S/W (no E); paging to a
connectionless edge drops that arrow and it returns on the way back.

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