[EverNife]

Problem

Mods like Et Futurum Requiem backport spectator mode by registering a GameType with id 3, and on plain Forge /gamemode 3 and the F3+F4 switcher work. On Crucible they don't:

• Crucible replaces vanilla /gamemode with Bukkit's GameModeCommand, which only knows survival/creative/adventure. /gamemode 3 and /gamemode spectator silently set the player to survival.
• Plugins built against 1.8+ APIs reference GameMode.SPECTATOR directly, so they fail with NoSuchFieldError on Crucible.
• If a mod puts a player in spectator by other means, Player#getGameMode() returns null.

Adding the constant at runtime with EnumHelper is not enough: it updates values()/valueOf(), but a plugin compiled with GameMode.SPECTATOR reads a static field, and that only exists if the constant is declared in the class.

Change

• GameMode gets a real SPECTATOR(3) constant, so newer plugins link on every Crucible server.
• CraftPlayer.isGameModeAvailable(GameMode) checks whether the server has a game type for the mode (GameType.getByID falls back to survival for unknown ids).
• CraftPlayer#setGameMode refuses an unavailable mode. The player keeps their current game mode, no event fires, and the console gets a warning that explains why.
• /gamemode accepts spectator/sp (and tab-completes it). When the mode is unavailable, it tells the sender so and suggests a mod that adds it.

Nothing is tied to a specific mod: any mod that registers game type 3 makes spectator available.

Testing

Crucible built from this branch (buildPackages, JDK 8), a probe plugin compiled against spigot-api 1.12.2, and a headless 1.7.10 client in each scenario, starting from survival:

Scenario	Without a spectator mod	With Et Futurum Requiem (master)
Plugin reads GameMode.SPECTATOR / switch on it	OK	OK
player.setGameMode(GameMode.SPECTATOR)	stays SURVIVAL, console warning	SPECTATOR on server and client
Console gamemode spectator <player>	stays SURVIVAL, red message to sender	SPECTATOR
Player runs /gamemode sp	stays SURVIVAL, red message in player chat	SPECTATOR

Console warning when no mod provides it:

Game mode of VanBot left unchanged at SURVIVAL: SPECTATOR is not available because no installed mod adds that game type. Install one that does, or pick another mode.

Side effect: GameMode.values() now always lists SPECTATOR, as on every 1.8+ server. Selecting it without a mod is refused as above.