docs, tests, benchmarks: improve trustworthiness and usability

[arthurpaulino]

Soundness & documentation:

• Add protocol documentation to prover and verifier modules
• Add full soundness argument to verifier with notation, per-component bounds (FRI, constraint folding, OOD, lookup, Fiat-Shamir), and overall soundness error formula
• Annotate each verification step with inline soundness comments
• Add security parameter docs to FriParameters
• Document unused InvalidClaim variant

Testing:

• Replace ignored 2^20-row benchmark test with fast 2^4-row test including serialization round-trip
• Add adversarial verifier tests: wrong claim, tampered stage-1 values, tampered accumulator, truncated proof, serialization round-trip
• Trim blake3 test_all_claims from 9 redundant runs to 6 minimal ones
• CI: run tests with --features parallel, add --all-features to clippy

Benchmarks & examples:

• Add Criterion benchmarks (prove/verify)
• Add examples: simple_proof (no lookups), preprocessed_proof (preprocessed trace with lookups), and lookup_proof (multi-circuit)

Repo cleanup:

• Rename chips/ to test_circuits/, move Blake3 helpers into blake3.rs
• Drop the "chip" terminology elsewhere
• Gate test_circuits module with #[cfg(test)]
• Add .cargo/config.toml with target-cpu=native
• Rewrite README with crypto setup, quick start, and soundness reference