Create arcane-affinity crate scaffold

[rebelmachina]

Epic: #64 (AffinityEngine)

What

Create the crates/arcane-affinity/ directory with:

• Cargo.toml (dependencies: arcane-core, uuid, tracing)
• src/lib.rs — public module declarations, AffinityEngine struct stub
• src/interaction_graph.rs — module stub
• src/scorer.rs — module stub
• src/hysteresis.rs — module stub
• src/config.rs — module stub

Add "crates/arcane-affinity" to workspace Cargo.toml members.

Acceptance criteria

• cargo build --workspace passes
• cargo test -p arcane-affinity passes (empty test suite is fine)
• Crate structure matches spec in in_08_affinity_engine.md

Spec

See arcane-engine repo: in_08_affinity_engine.md §7 (Dependencies/Crate dependencies)

[rebelmachina]

Implemented in PR #77. Crate scaffold created: crates/arcane-affinity/ with Cargo.toml, added to workspace Cargo.toml members, and registered as optional dep in arcane-infra under the affinity-clustering feature flag.