bugfixes and performance increase

[berisher]

Description

Brief summary of the changes in this PR.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test addition or update

Changes Made

• EXT and buffer opcodes now disabled by default, require explicit flags
• MemoIndexMutator excluded from fuzzer validation target
• Performance improvements: 10-20% faster across all benchmark categories

Testing Performed

Describe the tests you ran to verify your changes:

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass (cargo test)

Test commands:

cargo test
cargo bench  # if performance-related

Checklist

Before submitting this PR, please ensure:

• Code follows project style guidelines (cargo fmt)
• No clippy warnings (cargo clippy -- -D warnings)
• All tests pass (cargo test)
• Documentation updated (if applicable)
• CHANGELOG.md updated (if applicable)
• No breaking changes (or documented if necessary)
• Commit messages follow conventional commit format

Performance Impact

If this PR affects performance:

• Benchmarks run and results documented
• No significant performance regression
• Performance improvement quantified

Benchmark results:

see BENCHMARKS.md