DemoAgent: group_id field, GroupState, EncounterTracker, create_grouped_agents, tick_grouped_agents

[rebelmachina]

Part of epic #112.

Depends on: nothing — standalone change in arcane-demos.

Change

All changes in crates/arcane-demo/src/cluster_demo.rs (and exported from src/lib.rs):

1. Add group_id: Option<Uuid> to DemoAgent (None = ungrouped, identical to current behaviour). create_demo_agents continues to set group_id: None.

2. Add GroupState struct:

pub struct GroupState {
    pub centroid_x: f64,
    pub centroid_z: f64,
    pub encounter_target: Option<(f64, f64)>,
    pub encounter_ticks_remaining: u32,
}

3. Add EncounterTracker struct with new(), tick(), is_in_encounter(group_id), start_encounter(group_id, tick). Tracks per-group encounter cooldowns across ticks using a HashMap<Uuid, u32>.

4. Add create_grouped_agents(count, group_size, stress_radius) -> Vec<DemoAgent> — assigns consecutive agents to groups; group UUID is Uuid::from_u128(group_index as u128); group members spawn adjacent on the same circle as create_demo_agents.

5. Add compute_group_states(agents, tracker) -> HashMap<Uuid, GroupState> — computes centroid from positions of agents sharing a group_id; fills encounter fields from tracker.

6. Add tick_grouped_agents(agents, tick, group_states, tracker, stress_radius) — for agents with group_id = Some(...): blend direction 60% toward group centroid / 40% personal random; if in encounter, steer toward encounter target centroid. Agents with group_id = None behave identically to tick_demo_agents. Encounter triggers deterministically from group seed + tick.

Acceptance criteria

• create_demo_agents + tick_demo_agents signatures and behaviour are byte-for-byte identical to before
• Agents with group_id: None ticked via tick_grouped_agents produce same distribution as tick_demo_agents
• Test A: grouped agents have smaller average intra-group spread than ungrouped after 200 ticks
• Test B: encounter steering moves two groups closer together over 100 ticks
• Test C: ungrouped agents unaffected when passed through tick_grouped_agents
• cargo test -p arcane-demo passes
• cargo clippy -- -D warnings passes

Spec

See §3, §4.1–4.6, §5.1 in demo_agent_group_behaviour.md.

[rebelmachina]

Implemented in brainy-bots/arcane-demos PR #5. DemoAgent gets group_id: Option, GroupState, EncounterTracker, create_grouped_agents, compute_group_states, tick_grouped_agents all added to cluster_demo.rs. Existing APIs unchanged. Tests A/B/C pass.