fix: verify the actual exported inference artifact

[rylinjames]

Fixes #267.

• loads the declared, integrity-checked ONNX artifacts for the optimized arm
• binds the native arm to the exact exported model family and source reference
• rejects unsupported or conditioning-loss backends instead of falling back
• preserves camera masks and validates Pi0.5 secondary artifact paths
• adds protected-main artifact-execution receipts and real ONNX regressions

Independent review: APPROVE after three remediation rounds. Focused verifier suite, Ruff, compile checks, and diff checks pass.

[rylinjames]

Independent review: APPROVE after three remediation rounds. All security/correctness blockers were resolved; 60 focused tests, Ruff, compile checks, the protected quality ratchet, full pytest matrix, doctor matrix, and wheel smoke are green.