[Contract] Guild Member Inactivity Pruning Logic

[gabito1451]

Independence Note: Implement specifically inside isolated tests without wiping real instances.

Context:
In communities with thousands of members, we need a way to track if someone has been inactive for years to potentially prune them from governance voting arrays.

Objective:
Track the 'Last Active' ledger sequence for each member.

Acceptance Criteria:

• In the Guild Membership struct, add a field last_active_at (u64 sequence).
• Update this field whenever the member performs any action (votes, applies for bounty).
• Implement is_inactive(address) which returns true if no action in > 1,000,000 ledgers.
• Add unit tests simulating activity and verifying the inactivity flag changes correctly.

Technical Pointers:

• Recording a u64 is incredibly cheap and provides foundational data for future DAO optimizations.

[zp6]

PR submitted: #2 (zp6#2). Wallet: zp6