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/scripts/fish.ts

@@ -0,0 +1,45 @@
+import type { RomReader } from "../rom/reader";
+import { OVERWORLD } from "../rom/profile";
+
+/** The fishing rods' encounters (engine/events/fish.asm and
+ * data/wild/fish.asm). */
+export interface FishMon {
+  species: number;
+  level: number;
+}
+
+/** FISHGROUP_DATA_LENGTH: the bite chance, then a table per rod. */
+const FISHGROUP_DATA_LENGTH = 7;
+/** NITE_F: TimeFishGroups' second species from the night on. */
+const NITE_F = 2;
+
+/** Fish: rod 0 (Old), 1 (Good) or 2 (Super) in FishGroups entry `group`
+ * (FISHGROUP_*, never FISHGROUP_NONE). A bite when Random falls below the
+ * group's chance, then the first entry whose chance a second Random does
+ * not exceed; species 0 picks the time of day's TimeFishGroups entry.
+ * Undefined when nothing bites. GetFishGroupIndex's swarm groups need
+ * DAILYFLAGS1_FISH_SWARM_F and wFishingSwarmFlag, which only trainer calls
+ * set; they are not ported, so the map's own group is used. */
+export function fish(
+  r: RomReader,
+  group: number,
+  rod: number,
+  timeOfDay: number,
+  random: () => number,
+): FishMon | undefined {
+  const entry = OVERWORLD.FishGroups + (group - 1) * FISHGROUP_DATA_LENGTH;
+  if (random() >= r.byte(entry)) return undefined;
+  let p = r.near(entry + 1 + rod * 2, r.bank(entry));
+  const roll = random();
+  while (roll > r.byte(p)) p += 3;
+  let species = r.byte(p + 1),
+    level = r.byte(p + 2);
+  if (!species) {
+    // .TimeEncounter: the level byte indexes TimeFishGroups.
+    const time =
+      OVERWORLD.TimeFishGroups + level * 4 + (timeOfDay >= NITE_F ? 2 : 0);
+    species = r.byte(time);
+    level = r.byte(time + 1);
+  }
+  return { species, level };
+}