Add confidence-based verification to FRI protocol

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This PR implements a new feature that allows users to specify their desired confidence level for FRI verification instead of manually choosing the number of queries. Users can now say "I want to be sure at 99% that the prover is right" and the system automatically calculates the required parameters.

Key Changes

New FriConfig struct for confidence-based configuration:

// Create configuration for 99% confidence
let config = FriConfig::new(0.99)?;
let num_queries = config.calculate_num_queries(); // Returns 7 queries

// Use in verification
let (decommitments, queries, actual_confidence) = 
    FriCodeLayer::fri_decommitment_phase_with_confidence(
        &config, domain_size, &fri_layers, &mut channel
    );

Smart query calculation based on soundness analysis:

• 90% confidence → 4 queries (93.75% actual confidence)
• 95% confidence → 5 queries (96.88% actual confidence)
• 99% confidence → 7 queries (99.22% actual confidence)
• 99.9% confidence → 10 queries (99.90% actual confidence)

The system uses the simplified soundness model where each query has approximately 1/2 probability of catching a cheating prover, so the soundness error is roughly (1/2)^k for k queries.

Enhanced examples and documentation:

• Updated main program demonstrates both traditional fixed-query and new confidence-based approaches
• New standalone example (cargo run --example confidence_verification) showing the feature
• Comprehensive test coverage for all new functionality
• Updated README with usage examples and confidence level mappings

Backward compatibility: All existing code continues to work unchanged. The original fri_decommitment_phase() method is preserved alongside the new fri_decommitment_phase_with_confidence() method.

Fixes #11.

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