feat(yellow): port SurfingPikachu behaviour

Yellow's IsSurfingPikachuInParty swaps the player's overworld sheet to
a Pikachu-on-a-surfboard when the party mon that knows SURF is a
Pikachu. The recomp was missing both halves of this: the sheet was
never extracted, and the engine had no seam for the swap.

Extraction: SurfingPikachuSprite (gfx/sprites/surfing_pikachu.2bpp)
loads outside SpriteSheetPointerTable via
LoadSurfingPlayerSpriteGraphics2, the same bypass RedBikeSprite uses.
Added the symbol to the Yellow manifest and a parallel extract in
RomExtractor / build_rom_data / extract/sprites, minting
SPRITE_SURFING_PIKACHU. PaletteFX colors it (player OBP palette, same
as the bike).

Engine: new field.playerSprites.surfPikachu (default
SPRITE_SURFING_PIKACHU, guarded so before extraction the ride keeps
the Seel). Player.new caches surfPikachuSprite; pose() picks it when
surfing and the SURF-mon is a Pikachu. New
OverworldState:syncSurfingPikachu derives the flag from partyKnows at
every surf-state toggle (mount, dismount, fly, teleport, blackout,
forced-surf tile, boot-restore). Runtime-only, re-derived at load so a
party change between save and load is honored.

Lane B: RFC at docs/rfcs/0001-surfing-pikachu-sprite.md.
Backward-compatible — existing mods see no change (surf still defaults
to SPRITE_SEEL; surfPikachu only resolves on a Yellow import after
regeneration). Parity tests in tests/parity_surfing_pikachu_sprite.lua
(12/12) and tests/mod_world_tests.lua (19229/19229 with the new
boot-seed checks). tests/parity_cinnabar_east_surf.lua (24/24)
unchanged.

Regeneration required: re-run make_yellow_manifest.py against a
pret/pokeyellow checkout, then re-import the Yellow ROM.

src/world/FieldDefaults.lua

@@ -77,6 +77,12 @@ local SAFARI = {
 local PLAYER_SPRITES = {
   walk = "SPRITE_RED", surf = "SPRITE_SEEL",
   bike = "SPRITE_RED_BIKE", fly = "SPRITE_BIRD",
+  -- Yellow's IsSurfingPikachuInParty: when the SURF-mon is a Pikachu,
+  -- the player rides this sheet.  GFX loads via
+  -- LoadSurfingPlayerSpriteGraphics2, not SpriteSheetPointerTable, so
+  -- it needs a manifest extract (RFC 0001).  Guarded in Player.new so
+  -- before extraction lands the ride keeps the Seel.
+  surfPikachu = "SPRITE_SURFING_PIKACHU",
 }
 
 -- The player's own trainer art: RedPicBack (the battle back pic, up until

src/world/OverworldController.lua

@@ -389,6 +389,10 @@ function OverworldState:setMap(mapId, x, y, facing, opts)
         and not self.map:isWalkableCell(x, y)
         and self.map:isWaterCell(x, y)
     end
+    -- re-derive from the live party: a reloaded save with the SURF-Pikachu
+    -- since deposited should not render the Pikachu sheet.
+    -- ponytail: re-derived rather than persisted.
+    self:syncSurfingPikachu()
   end
   -- crossConnection re-arms this after setMap; clear so a warp/reload
   -- cannot leave a stale deferred PlayMapMusic pending
@@ -1484,6 +1488,21 @@ function OverworldState:partyKnows(moveId)
   return partyKnowsVanilla(moveId)
 end
 
+-- IsSurfingPikachuInParty (home/map_objects.asm): when the SURF-mon
+-- is a Pikachu, pose() renders the Pikachu surf sprite.  Called at
+-- every surf-state change so a reloaded save picks the right sheet
+-- after a party change.  No-op when not surfing.
+function OverworldState:syncSurfingPikachu()
+  local p = self.player
+  if not p then return end
+  if not p.surfing then
+    p.surfingPikachu = false
+    return
+  end
+  local mon = self:partyKnows("SURF")
+  p.surfingPikachu = mon ~= nil and mon.species == "PIKACHU" or false
+end
+
 -- The rejection loop shared by the Good and Super Rods
 -- (item_effects.asm ItemUseGoodRod .RandomLoop / ReadSuperRodData): an
 -- odd random byte is no bite; otherwise a 2-bit pick rerolls until it
@@ -1580,6 +1599,7 @@ function OverworldState:flyTo(mapId)
   Game.save.onBike = false
   Game.save.forcedBike = nil -- HandleFlyWarpOrDungeonWarp res BIT_ALWAYS_ON_BIKE
   self.player.surfing = false
+  self:syncSurfingPikachu()
   -- the bird carries the player off westward before the warp
   -- (engine/overworld/player_animations.asm LoadBirdSpriteGraphics)
   self.flyAnim = { frames = 48 }
@@ -1607,6 +1627,7 @@ function OverworldState:beginTeleportOut(onDone)
   end
   require("src.core.Sound").play(Game.data, "Teleport_Exit1")
   self.player.surfing = false
+  self:syncSurfingPikachu()
   self.player.inputLocked = true
   -- rising spin: the mirror of the arrival spin-drop set in startWarpTo, so
   -- spinRise lifts the sprite (Player:pose) while spinFrames counts down
@@ -2270,6 +2291,7 @@ function OverworldState:trySurf(fx, fy, onClose)
   Game.stack:push(TextBox.new(Game, text, function()
     if onClose then onClose() end
     p.surfing = true
+    self:syncSurfingPikachu()
     require("src.core.Music").setSurfing(Game.data, true)
     Game.stack:push(require("src.render.Transition").whiteFlash(Game, nil,
       function() self:stepForwardOrCrossEdge(p.facing) end))
@@ -3154,6 +3176,7 @@ function OverworldState:onStepComplete()
   -- dismounting a surf: landing on a walkable cell ends it
   if p.surfing and self.map:isWalkableCell(p.cellX, p.cellY) then
     p.surfing = false
+    self:syncSurfingPikachu()
     require("src.core.Music").setSurfing(Game.data, false)
   end
 
@@ -3455,6 +3478,7 @@ function OverworldState:checkForcedMovement()
         end
       elseif tile.mode == "surf" then
         p.surfing = true
+        self:syncSurfingPikachu()
         require("src.core.Music").setSurfing(Game.data, true)
       end
       return false
@@ -3733,6 +3757,7 @@ end
 function OverworldState:warpToHealPoint(onDone, opts)
   local heal = self:healPoint()
   self.player.surfing = false
+  self:syncSurfingPikachu()
   -- HandleFlyWarpOrDungeonWarp + DisplayPlayerBlackedOutText both clear
   -- BIT_ALWAYS_ON_BIKE (home/overworld.asm / home/text_script.asm)
   Game.save.forcedBike = nil

src/world/Player.lua

@@ -41,11 +41,18 @@ function Player.new(data, cx, cy, facing)
   -- LoadSurfingPlayerSpriteGraphics, home/overworld.asm)
   local walkId = FieldDefaults.fieldValue(data, "playerSprites", "walk")
   local surfId = FieldDefaults.fieldValue(data, "playerSprites", "surf")
+  local surfPikaId = FieldDefaults.fieldValue(data, "playerSprites", "surfPikachu")
   local bikeId = FieldDefaults.fieldValue(data, "playerSprites", "bike")
   self.sprite = SpriteRenderer.new(data.sprites[walkId], "player")
   if surfId and data.sprites[surfId] then
     self.surfSprite = SpriteRenderer.new(data.sprites[surfId], "player")
   end
+  -- Yellow's surfing-Pikachu ride (Yellow LoadSurfingPlayerSpriteGraphics2,
+  -- paired with field.playerSprites.surfPikachu). rotated in at pose()
+  -- when the SURF-mon is a Pikachu.
+  if surfPikaId and data.sprites[surfPikaId] then
+    self.surfPikachuSprite = SpriteRenderer.new(data.sprites[surfPikaId], "player")
+  end
   if bikeId and data.sprites[bikeId] then
     self.bikeSprite = SpriteRenderer.new(data.sprites[bikeId], "player")
   end
@@ -288,6 +295,7 @@ function Player:pose()
   -- RodResponse (engine/items/item_effects.asm) zeroes wWalkBikeSurfState
   -- across FishingAnim, so casting from the water shows the on-foot sheet
   local sprite = (self.fishing and self.sprite)
+                 or (self.surfing and self.surfingPikachu and self.surfPikachuSprite)
                  or (self.surfing and self.surfSprite)
                  or (self.onBike and self.bikeSprite) or self.sprite
   return sprite, self.px, py, facing, phase, flip, hopping