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>

docs/PLAN.md

@@ -169,7 +169,7 @@ The numbered sections below keep their full scope. Each one opens with a checkli
 - PACK: the Items, Balls, Key and TM pockets (left and right change pockets; there are no TMs or HMs yet), and each item's submenu chosen by its permissions and field menu (`USE`, `GIVE`, `TOSS`, `SEL`, `QUIT`). TOSS asks how many and confirms. USE follows `UseItem`: Oak's warning without a field use, "You don't have a POKéMON!" for party items without one, and `_DoItemEffect` by the item's `ItemEffects` address: Potions and the other `HealingHPAmounts` items, status healers, Full Restore, Revive and Max Revive, the bitter Energypowder, Energy Root, Heal Powder and Revival Herb, Ethers, Elixers, the MysteryBerry and PP Up. The Bicycle closes the menu and runs `Script_GetOnBike` or `Script_GetOffBike` outdoors; elsewhere Oak objects, because `wFieldMoveSucceeded` shares `wItemEffectSucceeded`'s byte.
 - PP Up keeps a quirk: `wUsePPUp` shares `wTempPP`'s byte, so `ComputeMaxPP` adds one step (a fifth of the base PP, at most 7) to the current PP as well as the maximum. Sketch and moves with three PP Ups are "maxed out".
 - The menu runs on a script draft, like the PC. Changes commit when it closes; a save made while it is open keeps the state from before it. The start menu cursor, the party menu cursor and the last pocket stay in memory between openings and are cleared after a battle, as `CleanUpBattleRAM` does. The party cursor reaches the next battle's `ChangeHappiness`.
-- Not ported: MAIL, switching to another party member's moves with left and right, item reordering with Select and the fishing rods (each gives a notice and changes nothing; the Escape Rope runs since the Fly section below, Repels and the other items of the last section since then). Rare Candy, evolution stones and vitamins run since the evolution section below; field moves, SEL and Select since the section after it.
+- Not ported: MAIL, switching to another party member's moves with left and right, and item reordering with Select (each gives a notice and changes nothing; the Escape Rope runs since the Fly section below, Repels and the other items of the last section since then). Rare Candy, evolution stones and vitamins run since the evolution section below; field moves, SEL and Select since the section after it.
 
 ## Implemented: evolution
 
@@ -248,6 +248,8 @@ The numbered sections below keep their full scope. Each one opens with a checkli
 - The Normal Box and the Gorgeous Box hold the silver and the gold trophy doll (`SetSpecificDecorationFlag`), sent home for the bedroom PC.
 - The Itemfinder answers "Yes! ITEMFINDER indicates there's an item nearby." when a hidden item not found yet lies on the original's screen around the player (`CheckForHiddenItems`: four squares left and up, five right, four down), and "Nope!" otherwise. Route 30's hidden Potion is the only one on an open map.
 - The Card Key and the Basement Key run their door scripts only facing the Radio Tower 3F slot or the Goldenrod Underground door (Oak objects elsewhere). The Squirtbottle sprinkles water, and on Route 36 starts the Sudowoodo battle when facing it (`WateredWeirdTreeScript`). None of these maps is open yet.
+- The Old, Good and Super Rods fish facing water, never while surfing (Oak objects otherwise; the facing tile may hold an object, as in the original). The map header's fish group decides: none gives "Not even a nibble!" at once; otherwise `Fish` bites below 50% + 1 and picks the rod's `FishGroups` entry, with `TimeFishGroups` for the morning and day or the night. The player casts with the fishing frames (`LoadFishingGFX`'s body and rod tiles), jiggles four times on a bite, shows "!", and after "Oh! A bite!" puts the rod away before a `BATTLETYPE_FISH` battle: "The hooked MAGIKARP attacked!". Qwilfish and Remoraid swarms come from trainer calls, which are not ported. New Bark Town, Route 30 and Cherrygrove City have fish.
+- Wild Magikarp now battle: `LoadEnemyMon` draws their DVs again as the original does, through `CalcMagikarpLength` (compared with the original routine) and its filters, whose millimetre limits only work as a floor of four feet that about 39% of draws skip (39 percent + 1), nowhere in map group 9 or on maps numbered 6 (the Lake of Rage check). Unown are still refused.
 - Save format 3 gained `repel`, `coins` and `blueCardBalance`, migrated by absence to 0.
 
 ## Native battles: current state