Implement MigrationState (hysteresis / cooldowns)

[rebelmachina]

Epic: #64 (AffinityEngine)
Depends on: #65 (crate scaffold)

What

Implement MigrationState in crates/arcane-affinity/src/hysteresis.rs:

• Per-entity cooldown tracking via HashMap<Uuid, u32>
• Methods: new(), record_migration(), is_on_cooldown(), tick(), remove_entity(), cooldown_count()
• tick() decrements all cooldowns by 1, removes entries at 0

Acceptance criteria

• All methods implemented
• Unit tests for: new state empty, record sets cooldown, is_on_cooldown works, tick decrements, expiry at exactly cooldown_ticks, remove_entity clears

Spec

See arcane-engine repo: in_08_03_hysteresis.md

[rebelmachina]

Implemented in PR #77. MigrationState (hysteresis) in crates/arcane-affinity/src/hysteresis.rs: per-entity cooldown map, tick() via retain + saturating_sub, is_on_cooldown(), record_migration(). 5 unit tests including exact cooldown expiry.