Feature: Add player and server data container systems with JSON persistence

[Copilot]

Adds per-player and server-wide JSON-backed data containers with automatic load/save lifecycle hooks, following the existing registration patterns.

New: data package

• PlayerData — open class with typed getString/getInt/getDouble/getBoolean getters and set/remove/has mutators over a JsonObject. Extend to expose strongly-typed Kotlin properties.
• PlayerDataManager — singleton; registers an internal Listener that loads playerdata/<uuid>.json on PlayerJoinEvent and flushes on PlayerQuitEvent. saveAll() is called from onDisable as a safety net. Supports a custom factory via setFactory { uuid -> MyPlayerData(uuid) }.
• ServerData / ServerDataManager — same API, single instance; loaded on init, persisted via save() from onDisable. Stored at serverdata.json.

Both managers log WARNING on corrupt/missing files and SEVERE on write failures.

Usage

// Basic — no subclassing required
val data = PlayerDataManager.get(player)
data.set("kills", data.getInt("kills") + 1)

// Typed subclass
class MyPlayerData(uuid: UUID) : PlayerData(uuid) {
    var kills: Int
        get() = getInt("kills")
        set(value) = set("kills", value)
}

PlayerDataManager.setFactory { uuid -> MyPlayerData(uuid) }  // before init
val typed = PlayerDataManager.get(player) as MyPlayerData
typed.kills++

Main.kt

• onEnable: calls ServerDataManager.init(this) then PlayerDataManager.init(this).
• onDisable: calls PlayerDataManager.saveAll() and ServerDataManager.save().

Docs

DEVELOPER_GUIDE.md updated with full Player Data and Server Data sections covering API reference, subclass pattern, and example listener/command.