Highlight differences & strengths of echidna vs. other fuzzers (like foundry)

Some things that could be covered:
- ease-of-use/difficulty in setting up the framework
- compatibility with solidity versions
- framework's areas of focus (echidna is e.g. better at property/invariant-testing whereas foundry is great for scripting/deployments & unit-tests)
- create a baseline of different fuzz/invariant tests that should be found and benchmark these

No file or test is named. Start by locating the repository’s existing guidance on Echidna, Foundry, fuzzing, and invariant testing, then define the comparison scope and Solidity versions to cover. Done means a documented comparison plus a reproducible baseline of fuzz and invariant tests with benchmark results.

Written by the indexing model from the issue text.