feat(settlement): optionally block settlement on unhealthy oracle heartbeat

[Oyinkans0la12]

Closes #188

Summary

Add an optional heartbeat-based settlement policy that allows administrators to prevent resolve_round from executing when the oracle heartbeat reports a degraded or unhealthy state.

Changes

• Added heartbeat status policy evaluation to resolve_round

• Added administrator-configurable strict/lenient enforcement

• Added blocked-settlement event with reason

• Added test coverage for all heartbeat status branches

Policy

Heartbeat Status | Lenient Mode | Strict Mode -- | -- | -- Healthy | ✅ Allow | ✅ Allow Degraded | ✅ Allow | ❌ Block Unhealthy | ✅ Allow | ❌ Block

Why

Settlement quality depends on the health of the oracle. This change allows deployments that require stronger guarantees to block settlement whenever the oracle heartbeat indicates degraded or unhealthy service, while preserving existing behavior in lenient mode.

Testing

Covered:

• Healthy heartbeat

• Degraded heartbeat (strict and lenient)

• Unhealthy heartbeat (strict and lenient)

• Blocked-settlement event emission

• Admin policy toggle behavior

Validation

Executed:

cargo test -p <affected-crate>
cargo clippy --workspace --all-targets -- -D warnings
cargo build --target wasm32-unknown-unknown --release

Scope

This PR intentionally focuses only on optional heartbeat-based settlement enforcement. It does not redesign the oracle subsystem, heartbeat implementation, settlement architecture, or event framework.

[Oyinkans0la12]

Conflicts resolved, please merge.