fix(network-monitor): move local proving off async runtime

[Ollie202]

Partially addresses #1976.

Summary

The network monitor performed local transaction execution and proof generation directly from async code in two places: counter account deployment and periodic counter increment submission. Both paths can run CPU-heavy VM/prover work without yielding and starve Tokio worker threads.

This PR moves both execution/proving paths into spawn_blocking_in_current_span. For increments, the blocking closure now constructs its own MonitorDataStore from the current block header, wallet account, and counter account. This avoids cloning or maintaining duplicate datastore state in TxBuilder.

Testing

• cargo +nightly fmt --all --check -- --config-path .config/rustfmt.toml
• cargo xtask fmt-comments --check --rustfmt-config .config/rustfmt.toml
• git diff --check upstream/next...HEAD
• GitHub CI: build, tests, lints, docs, stress test, WASM targets, and all Docker builds

A local cargo check -p miden-network-monitor --locked is blocked on Windows by the repository's proto build include-path handling; the Linux CI build and test jobs pass.

Changelog

changelog = "none"
reason    = "Internal async-runtime refactor only; no public interface or released behavior changes."

[Ollie202]

@SantiagoPittella following up on your two review comments — both have been addressed in the latest push (the deploy path no longer uses spawn_blocking and the changelog entry has been dropped). The changelog check is now red because the diff no longer touches CHANGELOG.md. Per your review, this PR doesn't need an entry — could you apply the no changelog label so the check turns green? Then this should be ready for another look. Thanks!