feat(mesh): upgrade to v0.74 and validate Buzz replies

[micspiral]

Summary

• upgrade embedded desktop and relay test dependencies to Mesh-LLM v0.74.0
• retain Gemma 4 as the curated non-reasoning default for reliable agent tool use
• extend the hardware harness with an exact buzz messages send assertion and accepted write response
• repair the Mesh-only recovery fixture for the shared agent field

Validation

• cargo check -p buzz-relay --example mesh_agent_e2e
• cargo test --manifest-path desktop/src-tauri/Cargo.toml --features mesh-llm mesh_llm::catalog::tests --lib (5 passed)
• cargo build --release -p buzz-agent -p buzz-dev-mcp
• live Gemma hardware harness: pending

Why draft

The dependency and focused unit/compile gates pass. Keep this draft until the v0.74 Gemma hardware run proves chat, tool use, and the Buzz reply command end to end.