feat(bench): add native FIDES and bounded task isolation

[arsenyinfo]

Summary

• add a distinct FIDES-native arm using automatic hiding and the official quarantine tools, with serialized middleware invocation to prevent thread-local cross-sample races
• pin the compatible OpenAI 2.x transport and add a timeout-shape regression test so provider failures cannot silently enter the prior retry loop
• extend guarded memory delegation with optional bounded scalars, fixed-unit measurements, and low-bandwidth compounds
• discard model response prose and deliver only deterministic field=value serialization of engine-attested known=true fields
• keep run manifests, audit validation, rendering, smoke inventory, and tests in sync with the fifth arm

Validation

• ruff format --check src/appa_agentthreatbench tests
• ruff check src/appa_agentthreatbench tests
• pytest (51 passed)
• uv lock --check
• runtime leakage audit: no upstream Memory Poisoning expected-answer string occurs in tasks.py
• three final-commit Memory Poisoning seeds:
  • standard: 86.7% ± 5.8 utility, 100% security
  • chaos: 76.7% ± 5.8 utility, 100% security
  • discarded model prose: 56.7% standard / 53.3% chaos utility

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