refactor(bin/arcane_swarm): extract BackendRuntime trait + implementations and control mode

[claude]

Summary

Extracts the BackendRuntime trait and its implementations (SpacetimeRuntime, ArcaneRuntime) from main.rs into a dedicated runtime.rs module, and moves control mode orchestration logic into control_mode.rs.

• main.rs reduced from 714 to 186 lines (74% reduction, well under 350 line requirement)
• Improves code navigation and separation of concerns
• No functional changes to behavior or wire formats

Changes

New Modules

• runtime.rs: Contains BackendRuntime trait + SpacetimeRuntime + ArcaneRuntime implementations with 6 unit tests
• control_mode.rs: Contains run_control_mode function and TCP control server logic

Refactored

• main.rs: Now focuses solely on argument parsing and main orchestration loop
• spawn_context.rs: Updated trait reference to use crate::runtime::BackendRuntime

Testing

• ✅ cargo build passes cleanly
• ✅ cargo test passes all 8 tests (6 new runtime tests + 2 existing tests)
• ✅ cargo fmt --all -- --check passes

Acceptance Criteria Met

• ✅ main.rs is 186 lines (< 350)
• ✅ BackendRuntime trait + impls in runtime.rs (can optionally be organized as runtime/ directory in future)
• ✅ 6 unit tests covering runtime module (exceeds 4 test minimum)
• ✅ cargo build + cargo test pass
• ✅ cargo fmt clean

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