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

pilot/manifest-supply.json

@@ -6,6 +6,7 @@
    "name": "image",
    "title": "The image",
    "blurb": "Course I's layers, shipped as one unit - the thing whose name we must say precisely.",
+   "z": 0,
    "points": [
     [
      8.03,
@@ -107,6 +108,7 @@
    "name": "registry",
    "title": "Upstream registry",
    "blurb": "The managed registry CI pushes to - Azure Container Registry in the worked example.",
+   "z": 0,
    "points": [
     [
      31.21,
@@ -216,6 +218,7 @@
    "name": "mirror",
    "title": "The mirror",
    "blurb": "zot, a pull-through cache: the fleet asks it, it asks upstream once, everything after is local.",
+   "z": 0,
    "points": [
     [
      48.51,
@@ -305,6 +308,7 @@
    "name": "core",
    "title": "The node",
    "blurb": "Pulls local. Boot, reschedule and recovery all lean on the mirror being there.",
+   "z": 0,
    "points": [
     [
      75.83,
@@ -430,6 +434,7 @@
    "name": "tag",
    "title": "The tag",
    "blurb": "A promise - a mutable pointer anyone with push rights can move, and nothing records that it moved.",
+   "z": 0,
    "points": [
     [
      26.2,
@@ -539,6 +544,7 @@
    "name": "digest",
    "title": "The digest",
    "blurb": "A fact - sha256 of the content. Same bytes, same digest, forever. The digest does the pulling.",
+   "z": 0,
    "points": [
     [
      49.82,