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>

scripts/check-reference.mjs

@@ -192,6 +192,7 @@ const ramNames = {
   visitedSpawns: "wVisitedSpawns",
   tempWildMonSpecies: "wTempWildMonSpecies",
   coins: "wCoins",
+  bgMapMode: "hBGMapMode",
   blueCardBalance: "wBlueCardBalance",
   moneyTemp: "hMoneyTemp",
 };