Add a property test: every signature verifies, and never over changed bytes

[HackTuah]

BeamMCP.Signer.Ed25519.sign/2 is tested with fixed inputs. A property test would cover many inputs at once:

• For random bytes and a freshly generated key, the signature always verifies with :crypto.verify/5 against the matching public key.
• The signature never verifies over bytes that differ in any way.
• The key passed as a function, private_key: fn -> key end, signs identically to the same key passed as bytes.

What to change:

• Add StreamData as a test-only dependency: {:stream_data, "~> 1.4", only: :test}.
• Add a property test under test/beam_mcp/signer/. The existing ed25519_test.exs shows how keys are generated in tests (:crypto.generate_key(:eddsa, :ed25519)). No key is ever written to the tree.

Done when:

• mix test passes, including the new property.
• mix credo --strict and mix format --check-formatted are clean.
• You have shown the property can fail: break the signing in a throwaway copy (for example, sign different bytes), show the property turn red, then restore it. Mention this in the PR.

How to contribute: see CONTRIBUTING.md. Sign off your commit with git commit -s.