ci(argo): wire sccache into compile-and-deploy-template

Mount the existing sccache-cpu PVC and set RUSTC_WRAPPER=sccache for
the regular service-build pipeline. Previously only train-template and
train-multi-seed-template used sccache; the production CI build had
zero rustc-level caching, so every CI run rebuilt every dep from
scratch.

Also set CARGO_INCREMENTAL=0 to override the workspace-wide
[build] incremental = true in .cargo/config.toml. sccache documented
limitation: cannot cache incremental rustc output. Local dev keeps
incremental via config.toml unchanged.

Print sccache --show-stats at end of build for visibility into cache
hit rates.

Mirror of train-template.yaml's sccache pattern; uses the sccache-cpu
PVC (20Gi, scw-bssd-retain) which was already provisioned but unmounted.

Expected impact: rustc cache hit rate jumps from 0% → 50-90% on
subsequent CI runs.

infra/k8s/argo/compile-and-deploy-template.yaml

@@ -44,6 +44,9 @@ spec:
     - name: cargo-target-cpu
       persistentVolumeClaim:
         claimName: cargo-target-cpu
+    - name: sccache
+      persistentVolumeClaim:
+        claimName: sccache-cpu
 
   templates:
     - name: pipeline
@@ -179,6 +182,21 @@ spec:
             value: /cargo-target
           - name: CARGO_HOME
             value: /cargo-target/cargo-home
+          # sccache: rustc wrapper that content-hashes compile inputs and reuses
+          # object output across pods/commits via the sccache-cpu PVC. Survives
+          # cargo's incremental-cache invalidation from git checkout mtime touches.
+          - name: RUSTC_WRAPPER
+            value: sccache
+          - name: SCCACHE_DIR
+            value: /sccache-cache
+          - name: SCCACHE_CACHE_SIZE
+            value: 20G
+          # Required: workspace `.cargo/config.toml` sets [build] incremental = true,
+          # which forces incremental compilation even on --release.  sccache cannot
+          # cache incremental rustc output (documented limitation).  Override here
+          # so sccache actually catches.  Local dev keeps incremental via config.toml.
+          - name: CARGO_INCREMENTAL
+            value: "0"
           - name: GITLAB_PAT
             valueFrom:
               secretKeyRef:
@@ -197,6 +215,8 @@ spec:
             readOnly: true
           - name: cargo-target-cpu
             mountPath: /cargo-target
+          - name: sccache
+            mountPath: /sccache-cache
         args:
           - |
             set -e
@@ -312,6 +332,9 @@ spec:
                 "${GITLAB}/api/v4/projects/1/packages/generic/foxhunt-services/latest/${BIN_NAME}" || true
             done
 
+            echo "=== sccache stats ==="
+            sccache --show-stats || true
+
             echo "=== Service compile + upload done ($SERVICE_PKGS) ==="
 
     # ── upload-release: create GitLab Release with package links ──