Stop player save snapshots sharing mutable state with the player

[HarleyGilpin]

Partially addresses #1252. The atomic write half of that issue is untouched, so it stays open.

Player.copy() builds the snapshot that gets serialized on Dispatchers.IO. Every field is defensively copied except two:

variables = variables.data.toMap(),
inventories = inventories.instances.mapValues { it.value.items.map { itm -> itm.copy() }.toTypedArray() },
friends = friends,
ignores = ignores.toList(),
offers = offers.copyOf(),

friends is val friends: MutableMap<String, ClanRank> = mutableMapOf() (Player.kt:44), handed straight to the IO thread by reference. The game thread keeps writing to it: FriendsList.kt:67 and ClanChat.kt:106 both do player.friends[account.accountName] = .... Adding a friend during a save can throw ConcurrentModificationException while the file is being written.

offers.copyOf() copies the array but not the ExchangeOffer elements, and state, completed and coins are var, so a GE update mid-save tears the snapshot.

Neither fails loudly. The friends case throws on the IO thread after Config.fileWriter has truncated the target, which is the part that turns it into a corrupt save rather than a retried one. That interaction is why #1252 covers both.

friends = friends.toMap(),
offers = Array(offers.size) { offers[it].copy() },

ExchangeOffer is a data class whose remaining fields are val, so an elementwise copy() fully detaches it. ExchangeHistory is entirely val, so history.toList() was already safe and is left as is.

Both tests fail without the change:

Snapshot changed after the player edited their friends list. Expected <{first=Friend}>, actual <{first=Friend, second=Friend}>.
Snapshot changed after the player's offer progressed. Expected <0>, actual <7>.

Full suite green.