Add event for member join and leave actions

[WuXieSec]

This PR adds structured events for member join and leave actions, as requested in #53.

Changes:

• Emit mem_join event with (group_id, member, timestamp, member_count)
• Emit mem_leave event with (group_id, member, timestamp, member_count)
• Include member count after action in event data
• Add test coverage for event emission

Why this matters:
These events enable:

• Indexers to track group membership changes over time
• Real-time UI updates when members join/leave
• Analytics on group formation patterns
• Historical membership tracking

Event structure:

// Join event
("mem_join", (group_id: u64, member: Address, timestamp: u64, member_count: u32))

// Leave event
("mem_leav", (group_id: u64, member: Address, timestamp: u64, member_count: u32))

Test coverage:

• ✅ Verify mem_join event is emitted on join
• ✅ Verify mem_leave event is emitted on leave
• ✅ Verify member count is correctly tracked in events

Closes #53