feat(turns): add loop exit handshake contract

[serrrfirat]

Summary

• Add distinct driver-facing LoopExit contract types and bounded refs in ironclaw_turns.
• Add pure validation/mapping from LoopExit to trusted TurnRunnerOutcome or RecoveryRequired.
• Document the LoopExit handshake and link it from Reborn turn runner/agent-loop contracts.

Test Plan

• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target cargo test -p ironclaw_turns --test loop_exit_contract
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target cargo test -p ironclaw_turns --all-features --test loop_exit_contract
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target cargo test -p ironclaw_turns --test turn_coordinator_contract
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target cargo test -p ironclaw_turns --lib
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target cargo test -p ironclaw_turns --all-features --test db_turn_state_store_contract
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target cargo clippy -p ironclaw_turns --all-targets --all-features
• bash scripts/pre-commit-safety.sh
• cargo fmt --check --package ironclaw_turns
• git diff --check

Closes #3232

[gemini-code-assist]

Code Review

This pull request implements the LoopExit handshake contract in the ironclaw_turns crate, introducing a structured way for agent-loop drivers to report attempt outcomes. It adds new ID types, the LoopExit enum, and validation logic that translates driver claims into trusted runner outcomes or recovery states based on safety policies. The PR also includes extensive tests for the validation logic and new documentation detailing the contract's purpose and requirements. I have no feedback to provide.