Make Rust formatting guidance runnable from repo root

The Rust crate layout expects formatting to run from the rust directory, so add a root-level wrapper that preserves the working command while forwarding user flags like --check. Documentation now points contributors at the wrapper instead of the misleading virtual-workspace manifest invocation.

Constraint: Root-level cargo fmt --manifest-path rust/Cargo.toml is misleading for this virtual workspace
Rejected: Document cd rust && cargo fmt directly | a root wrapper gives one stable repo-root command
Confidence: high
Scope-risk: narrow
Tested: scripts/fmt.sh --check
Tested: git diff --check

scripts/fmt.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd "$(dirname "$0")/../rust"
+exec cargo fmt "$@"