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