Build modules run at most as many steps as their node's build CPU gives, and say a check's phases

Each of mesh-build-go, -node and -image declares cores-per-step (6, 2, 2) and hands it to the
SDK puller, which bounds the configured capacity by floor(slice quota cores / cores-per-step),
read live; thread limits and the status tools follow the effective capacity (issue 1806's
follow-up: three Go checks on shanks' 12-core slice starved past the 30-minute step bound).
mesh-build-go says its lint and test phases with the ask id (issue 1890).

modules/mesh-build-node/module.json

@@ -25,6 +25,12 @@
       "description": "How many asks of the Node build seat this module runs at once on each node, as node=asks pairs separated by commas, such as shanks=3,ace=2 (novox/hq ADR 0525 §5); a node set to 0 pulls nothing, a node not named runs one at a time. The control-node and the laptop build nothing.",
       "pattern": "|[a-z0-9_-]+=[0-9]+(,[a-z0-9_-]+=[0-9]+)*",
       "default": "shanks=3,ace=2,g14=0,novox=0"
+    },
+    "cores-per-step": {
+      "type": "number",
+      "description": "How many processors one Node step of this module needs (tsc, esbuild and a test runner; a median step takes 15 s). The capacity above is an upper bound: on each node the module runs at most floor(the cores of its build slice's CPU quota / this) steps at once, the quota read live from its cgroup (issue 1806), so a lowered build CPU share never starves its steps. Above 0.",
+      "minimum": 0.1,
+      "default": 2
     }
   },
   "tools": [
@@ -100,7 +106,7 @@
       "path": "${dir:state}/settings.json",
       "mode": "0600",
       "owner": "mesh-build-node",
-      "takes-config": ["capacity"],
+      "takes-config": ["capacity", "cores-per-step"],
       "merge": "json",
       "template": "files/settings.json"
     },