fix(argo): force-checkout in ensure-binary to handle dirty Cargo.lock

The cargo-target-cuda PVC persists `$BUILD` across runs. cargo build
mutates Cargo.lock, so the next run's `git checkout $SHA` fails with
"Your local changes to the following files would be overwritten by
checkout: Cargo.lock". Same pattern alpha-perception-template uses
(`git checkout --force $SHA; git clean -fd`) handles this cleanly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

infra/k8s/argo/lob-backtest-sweep-template.yaml

@@ -216,7 +216,13 @@ spec:
 
             echo "=== Cache MISS: compiling fxt-backtest for $SHORT_SHA ==="
             if [ -d "$BUILD/.git" ]; then
-              cd "$BUILD"; git fetch origin; git checkout "$SHA"
+              # Same pattern as alpha-perception-template.yaml: prior
+              # cargo build mutates Cargo.lock (and sometimes other
+              # generated files); --force checkout overwrites them,
+              # `git clean -fd` drops untracked. Without this, the
+              # second SHA's checkout fails on dirty working tree.
+              cd "$BUILD"; git fetch origin
+              git checkout --force "$SHA"; git clean -fd
             else
               git clone --filter=blob:none "$REPO" "$BUILD"
               cd "$BUILD"; git checkout "$SHA"