perf(runtime): migrate std::sync::Mutex to parking_lot where not held across await

[lgahdl]

Summary

Fixes #280. Migrated every std::sync::Mutex site the issue named to parking_lot::Mutex — faster under contention, one word instead of a boxed OS primitive, no lock poisoning.

Audited each site individually before migrating (the issue's own explicit caveat — a guard held across an .await must not become a parking_lot guard, since it's !Send and would block the executor thread):

• crates/nexum-runtime/src/host/logs/{mod,stdio,store}.rs — store.rs is real production code (InMemoryRunLogStore); mod.rs/stdio.rs sites are #[cfg(test)] mocks. All non-async or async-but-lock-dropped-before-any-.await.
• crates/nexum-runtime/src/test_utils/{chain,clock,store}.rs — test-utils-feature-gated mocks. chain.rs's async move { ... } blocks acquire the lock, do synchronous work, and return — never await while holding it. Verified directly, not inferred.
• crates/nexum-sdk/src/tracing.rs — the one Mutex here is inside #[cfg(test)] mod tests, so it's a [dev-dependencies] addition and never touches the actual wasm32 guest build.

Scope correction: crates/nexum-runtime/src/host/pool_router.rs, named in the issue alongside ledger/watched Mutex fields, no longer exists — grepped the whole tree for both, found neither. That part of the issue's scope is stale; nothing to migrate there.

parking_lot was already in the tree transitively (0.12.5, per cargo tree), so adding it as a direct dependency resolved to the same version — Cargo.lock diff is just two new dependency-edge lines, no version bumps.

Dropped the poison-handling boilerplate everywhere it touched (.unwrap(), .expect("... poisoned"), .unwrap_or_else(|e| e.into_inner())) since parking_lot::Mutex::lock() returns the guard directly.

The one existing tokio::sync lock is untouched, as scoped by the issue.

Test plan

• cargo build --release --target wasm32-wasip2 for all 12 guest modules — clean (confirms the nexum-sdk dev-dependency never reaches the wasm32 build)
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings — clean
• cargo fmt --all -- --check — clean
• cargo test --workspace --all-features --locked — full suite green