feat(architecture): implement phase 2 config decoupling

[Lythaeon]

Summary

• replace mutating config application with a builder-style return path
• centralize merge precedence in one place (CLI > Config > Preset defaults)
• refactor scenario parsing to consume explicit scenario defaults instead of TesterArgs
• update entry planning, distributed manual scenario resolution, wasm scenario loading, fuzzing helpers, and config tests

Testing

• cargo make format
• cargo make clippy
• cargo make test
• cargo make test-wasm