Sort party presences by leader first in party state

[Copilot]

This PR implements automatic sorting of party member lists to ensure the party leader is always the first element in the array. This makes the party state self-describing and enables reliable leader identification even when real-time API messages are missed.

Problem

Previously, there was no way to determine the party leader from the list of party members alone if RT messages were missed. This created friction for clients, plugins, and external systems that needed to identify the leader after joining an existing party.

Solution

The implementation adds a ListSorted() method to PartyHandler that returns party members with the leader guaranteed to be first. Key changes include:

• New PartyHandler.ListSorted() method: Uses copy-and-swap to sort members with leader first
• Updated party join flow: Client messages now include sorted member lists
• Updated EVR wrapper classes: PartyGroup.List() and LobbyGroup.List() return sorted lists
• Comprehensive test coverage: Validates sorting behavior including edge cases

Technical Details

The sorting uses O(n) linear search to locate the leader and swaps them to the front position. The implementation:

• Creates copies to avoid modifying original slices
• Uses proper read locking for thread safety
• Handles edge cases (no leader, single member, leader already first)
• Maintains existing API contracts with only ordering changes

Example

// Before: Leader could be anywhere in the list
members := partyHandler.members.List()
// [Alice, Bob(Leader), Charlie] - leader position unknown

// After: Leader is guaranteed to be first
members := partyHandler.ListSorted()  
// [Bob(Leader), Alice, Charlie] - leader always at index 0

This enables clients to reliably identify the leader as members[0] without depending on separate leader change notifications.

Fixes #86.

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