Add a deadline for accuracy scoring and finalization

Context

PR #409 consolidates the existing benchmark timeout configuration. accuracy_drain_timeout_s only bounds outstanding endpoint responses from the accuracy load phase. It does not cover score_accuracy() or finalization, and scorers with SKIP_ENDPOINT_PHASE bypass that response drain entirely.

The run watchdog is currently cancelled before finalize_benchmark(). External-service scorers such as SWE-bench and subprocess-based scorers such as VBench/LiveCodeBench can therefore wait without a benchmark-level deadline.

Scope

Add an explicit deadline for accuracy scoring and finalization in a follow-up MR. Define how it composes with run_timeout_s rather than silently changing the existing response-drain timeout.

Acceptance criteria

• Scoring/finalization has an explicit, documented deadline.
• External-service and subprocess scorers have defined cancellation/termination behavior.
• SKIP_ENDPOINT_PHASE scorers are covered.
• Expiry produces honest incomplete/interrupted artifacts and a non-zero exit.
• No scorer process or service is leaked after expiry.
• Integration coverage exercises service-backed and subprocess-backed timeout paths.

Start by reading PR #409 and tracing the run watchdog, score_accuracy(), finalize_benchmark(), accuracy_drain_timeout_s, and SKIP_ENDPOINT_PHASE paths. Map how run_timeout_s currently composes with scoring and finalization, then add integration coverage for service-backed and subprocess-backed scorers; done means expiry leaves honest incomplete artifacts, returns non-zero, and leaks no scorer process or service.