[FR-009] Orchestrator stores failed evals with error_class (no drop from denominator)

[explosivebit]

Trace: PRD prd-v0-1-smoke-evaluation-run · FR-009 · RFC error handling taxonomy · Invariant #1 (NEVER violated)

Capability: asyncio.gather(*..., return_exceptions=True) preserves failures. Each failed eval gets error_class from enum (network/rate_limit/5xx_server/4xx_client/timeout/contract_violation/sandbox_failure).

Acceptance:

• AC-5 from RFC: 4xx_client → 0 retries (fail-fast)
• AC-5 from SPEC: failed eval present in evals[] (NOT dropped) + counts_by_status.failed += 1
• SC-3 PRD: 0 dropped evals from denominator

Implementation locus: apps/eval-core-py/src/orchestrator/grid_runner.py