feat: event schema registry and circuit-breaker resume policy

[Abd-Standard]

Closes #601

Closes #606

#601 – EventSchemaRegistry in events.rs

• Add EventSchemaEntry {topic, schema_version} contracttype
• Add EventSchemaRegistry::get_schema(env, name) as single source of truth for oracle_result and dispute_created topics (v1)
• emit_oracle_result / emit_dispute_created now read topic+version from the registry instead of inline literals; topic tuple gains schema_version
• Tests: lookup hit, lookup miss, version mismatch, emit smoke tests

#606 – HalfOpen cooldown and admin resume in circuit_breaker.rs

• Add half_open_since: u64 to CircuitBreakerState (zero when not HalfOpen)
• Add CircuitBreaker::request_resume(env, admin): Open -> HalfOpen with cooldown; errors if breaker is not Open
• record_success: probe count only increments after recovery_timeout has elapsed since half_open_since; auto-closes on threshold
• record_failure: re-opens from HalfOpen and clears half_open_since
• Expose request_resume as a contract entrypoint in lib.rs
• Tests: cooldown-not-elapsed, probe-failure-reopens, probe-success-threshold-closes

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