Port the fishing rods and wild Magikarp

The Old, Good and Super Rods run FishFunction facing water: the map's fish
group and Fish pick no fish, no bite, or a Pokémon from FishGroups and the
time of day's TimeFishGroups. The player casts and jiggles with the
fishing frames drawn from LoadFishingGFX's tiles, shows "!" on a bite, and
the hooked Pokémon attacks as BATTLETYPE_FISH. Facing pieces now use
signed offsets, which the rod above and left of the player needs.

Wild Magikarp no longer stop a battle: LoadEnemyMon redraws their DVs
through CalcMagikarpLength and its filters, as the original does. The
length routine matches the original in the reference emulator.

Part of #27

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/game/battle/constants.ts

@@ -311,6 +311,7 @@ export const C = {
   BATTLETYPE_NORMAL: 0,
   BATTLETYPE_DEBUG: 2,
   BATTLETYPE_TUTORIAL: 3,
+  BATTLETYPE_FISH: 4,
   BATTLETYPE_ROAMING: 5,
   BATTLETYPE_CONTEST: 6,
   BATTLETYPE_FORCESHINY: 7,