arc: z-order in the slice engine + gitops collapse tune (shard and cluster nests)

pilot/manifest-gitops.json

@@ -6,6 +6,7 @@
    "name": "repo",
    "title": "The repository",
    "blurb": "Desired state lives here. The deploy button is a git commit; the audit log is git log.",
+   "z": 3,
    "points": [
     [
      13.7,
@@ -147,6 +148,7 @@
    "name": "engine",
    "title": "ArgoCD",
    "blurb": "Watches repo + path + revision, compares desired against actual, and syncs the difference.",
+   "z": 5,
    "points": [
     [
      48.29,
@@ -288,6 +290,7 @@
    "name": "sheets",
    "title": "Rendered manifests",
    "blurb": "What the repo's charts and manifests render to - build artefacts nobody hand-edits.",
+   "z": 2,
    "points": [
     [
      39.57,
@@ -397,6 +400,7 @@
    "name": "cluster",
    "title": "The cluster",
    "blurb": "Actual state. It pulls its own future - no CI system holds a credential that can touch it.",
+   "z": 1,
    "points": [
     [
      52.8,
@@ -535,13 +539,14 @@
      73.22
     ]
    ],
-   "dx": -20.96,
-   "dy": -19.23
+   "dx": -16.01,
+   "dy": -14.69
   },
   {
    "name": "shard",
    "title": "Drift",
    "blurb": "A hand-edited live object. selfHeal puts it back; what leaves git leaves the cluster.",
+   "z": 4,
    "points": [
     [
      80.04,
@@ -660,13 +665,14 @@
      94.73
     ]
    ],
-   "dx": -27.99,
-   "dy": -32.13
+   "dx": -13.61,
+   "dy": -15.62
   },
   {
    "name": "vault",
    "title": "The vault",
    "blurb": "Azure Key Vault in the worked example. Git carries the reference; the vault carries the value. Never absorbed into the pipeline.",
+   "z": 6,
    "points": [
     [
      56.07,