fix: cap pending connection backlog

[sanity]

Problem

Nodes were experiencing unbounded growth in pending connection targets, with pending_adds increasing indefinitely (e.g., 2831 → 2855 → 2879 → 2903 → 2927) even when unable to establish new connections.

From the logs:

INFO freenet::ring: Topology adjustment result, ... current_connections: 1, pending_adds: 2831
INFO freenet::ring: Topology adjustment result, ... current_connections: 1, pending_adds: 2855
INFO freenet::ring: Topology adjustment result, ... current_connections: 1, pending_adds: 2879

This represents a memory leak where the pending_conn_adds BTreeSet grows without bound.

Root Cause

The connection maintenance loop:

1. Can only process one pending connection at a time (gated by live_tx)
2. Topology adjustment adds ~24 new locations per iteration
3. No limit was enforced on pending additions based on existing backlog
4. Result: backlog grows faster than it can be processed

Solution

Added calculate_allowed_connection_additions() function that:

• Considers both current and pending connections as "effective connections"
• Enforces max_connections limit based on effective connections
• When below min_connections, limits additions to deficit needed
• Prevents backlog from satisfying the entire connection capacity

Example Scenarios

Healthy state (1 connection, 0 pending, need 25 min):

• Allows 24 additions to reach minimum

Backlog exists (1 connection, 24 pending, need 25 min):

• Blocks new additions since backlog will satisfy minimum
• Allows system to process existing backlog

At capacity (1 connection, 2831 pending, max 200):

• Blocks all new additions (effective = 2832 > max = 200)
• Prevents unbounded growth

Testing

• Added 4 unit tests covering key scenarios
• All existing ring tests pass
• Validates backlog is respected when calculating capacity

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[sanity]

[Codex] Change looks solid to me—thanks for capping the backlog and adding the targeted tests to guard the helper.