Implement AffinityConfig with defaults and env-var loading

[rebelmachina]

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

What

Implement AffinityConfig in crates/arcane-affinity/src/config.rs:

• All fields pub for benchmark parameter sweeps
• Default impl with production-ready values
• Optional: from_env() constructor for runtime configuration via environment variables

Key parameters

Parameter	Default	Purpose
decay_factor	0.97	Exponential decay per tick (~3.3s half-life at 10 Hz)
gc_threshold	0.001	GC prune threshold
weight_party_member	5.0	Same-party interaction weight floor
weight_guild_member	1.0	Same-guild interaction weight floor
weight_proximity_per_tick	0.1	Proximity-based weight per tick
proximity_radius	50.0	World units for proximity detection
spatial_weight	0.2	Blend factor (0=pure interaction, 1=pure spatial)
migration_threshold	3.0	Min score improvement to trigger migration
cooldown_ticks	50	Post-migration cooldown
merge_entity_threshold	5	Min entities wanting to move for a Merge decision

Acceptance criteria

• Struct with all fields and Default impl
• cargo test validates default values are within expected ranges

Spec

See arcane-engine repo: in_08_04_affinity_config.md

[rebelmachina]

Implemented in PR #77. AffinityConfig in crates/arcane-affinity/src/config.rs with all tunable fields and Default impl: decay_factor 0.97, gc_threshold 0.001, weight_party_member 5.0, weight_guild_member 1.0, weight_proximity_per_tick 0.1, proximity_radius 50.0, spatial_weight 0.2, migration_threshold 3.0, cooldown_ticks 50, merge_entity_threshold 5.