Run the scrub schedule as a scheduled process, never pause a scrub, and keep the pool's name the only setting in a unit

The second review of #147 found three blockers before the home server.

1. ${setting:} was not filled in a unit's name, so the distribution timers would have reached the
   machine as "zfs-scrub-weekly@${setting:scrub-pool}.timer". mesh-controller #171 fills it and refuses
   any value that is not a unit-name part. The module's own schedule needs no setting in a name now: the
   node-engine's timer for the scheduled process zfs-mesh-scrub fires `zfs-tools scrub-run` every quarter
   of an hour, and the run reads the pool, cadence, day and time from the configuration file.
2. The run does what upstream's zfs-scrub@.service does: if a scrub is running it waits for it with
   zpool wait -t scrub, otherwise it starts one and waits. It never pauses or stops a scrub, and the unit
   has no ExecStop. A resilver defers the scrub. The served slot is kept in /var/lib/zfs-tools.
3. Unassigning left the pool with no schedule. mesh-host #66 (hq ADR 0269, proposed) gives the
   distribution timer back running and enabled. The README says not to unassign before it is merged.

Also: the run comes before the distribution timers in the declaration; the day and time are checked
against fixed sets and reach no unit file (an invalid value runs nothing); a due scrub not started an
hour after its time is a warning; the error counts outlive a restart in a small state file.

modules/zfs/module.json

@@ -12,17 +12,17 @@
     "scrub-cadence": {
       "kind": "preference",
       "default": "weekly",
-      "why": "how often the pool named by scrub-pool is scrubbed, weekly or monthly: weekly is what the distribution's timer did on the home server before the module, so taking it over changes nothing; a pool whose scrub takes days may want monthly"
+      "why": "how often the pool named by scrub-pool is scrubbed, weekly or monthly (the first scrub-day of each month); weekly is what the distribution's timer did on the home server before the module, so taking it over moves nothing until the operator chooses"
     },
     "scrub-day": {
       "kind": "preference",
       "default": "Mon",
-      "why": "the weekday the scrub starts (Mon to Sun); for a monthly scrub, the first such day of the month. Monday is when the distribution's weekly timer fired"
+      "why": "the weekday the scrub starts, Mon to Sun: for a monthly scrub, the first such day of the month. Checked by the scheduled run, which refuses any other value and runs nothing; Monday is when the distribution's weekly timer fired"
     },
     "scrub-time": {
       "kind": "preference",
       "default": "00:00",
-      "why": "the time the scrub starts, HH:MM in the machine's time zone; midnight is when the distribution's weekly timer fired"
+      "why": "the time the scrub starts, HH:MM from 00:00 to 23:59 in the machine's time zone; the scheduled run checks it every quarter of an hour, so the scrub starts within 15 minutes of it. Midnight is when the distribution's weekly timer fired"
     }
   },
   "tools": [
@@ -114,6 +114,13 @@
         "grace": "60s"
       }
     },
+    {
+      "id": "state-dir",
+      "type": "directory",
+      "path": "/var/lib/zfs-tools",
+      "mode": "0755",
+      "owner": "${machine:account}"
+    },
     {
       "id": "cursor-dir",
       "type": "directory",
@@ -147,25 +154,15 @@
       "content": "# Written by the mesh (module zfs) from this machine's assignment. Replaced on every push; change the\n# settings scrub-pool, scrub-cadence, scrub-day and scrub-time, never this file. The module's tools read it.\npool=${setting:scrub-pool}\ncadence=${setting:scrub-cadence}\nday=${setting:scrub-day}\ntime=${setting:scrub-time}\n"
     },
     {
-      "id": "scrub-service",
-      "type": "file",
-      "path": "/etc/systemd/system/zfs-mesh-scrub@.service",
-      "mode": "0644",
-      "content": "# Written by the mesh (module zfs). Replaced on every push; change the module's scrub settings, never this file.\n# The scrub of one pool, started by the module's timer: zpool with fixed arguments and no shell. Stopping the\n# unit while a scrub runs pauses it, as the distribution's own scrub unit does; a later start resumes it.\n[Unit]\nDescription=zpool scrub of %i (the mesh's zfs module)\nDocumentation=man:zpool-scrub(8)\nRequires=zfs.target\nAfter=zfs.target\nConditionPathIsDirectory=/sys/module/zfs\n\n[Service]\nExecStart=/usr/bin/zpool scrub -w -- %i\nExecStop=-/usr/bin/zpool scrub -p -- %i\n"
-    },
-    {
-      "id": "scrub-weekly",
-      "type": "file",
-      "path": "/etc/systemd/system/zfs-mesh-scrub-weekly@.timer",
-      "mode": "0644",
-      "content": "# Written by the mesh (module zfs). Replaced on every push; change the module's scrub settings, never this file.\n# The weekly scrub schedule. The mesh enables it for the pool its scrub-pool setting names, and holds the\n# distribution's own scrub timers for that pool stopped and disabled, so one schedule scrubs the pool.\n[Unit]\nDescription=Weekly zpool scrub of %i (the mesh's zfs module)\nDocumentation=man:zpool-scrub(8)\n\n[Timer]\nOnCalendar=${setting:scrub-day} *-*-* ${setting:scrub-time}:00\nPersistent=true\nUnit=zfs-mesh-scrub@%i.service\n\n[Install]\nWantedBy=timers.target\n"
-    },
-    {
-      "id": "scrub-monthly",
-      "type": "file",
-      "path": "/etc/systemd/system/zfs-mesh-scrub-monthly@.timer",
-      "mode": "0644",
-      "content": "# Written by the mesh (module zfs). Replaced on every push; change the module's scrub settings, never this file.\n# The monthly scrub schedule. The mesh enables it for the pool its scrub-pool setting names, and holds the\n# distribution's own scrub timers for that pool stopped and disabled, so one schedule scrubs the pool.\n[Unit]\nDescription=Monthly zpool scrub of %i (the mesh's zfs module)\nDocumentation=man:zpool-scrub(8)\n\n[Timer]\nOnCalendar=${setting:scrub-day} *-*-01..07 ${setting:scrub-time}:00\nPersistent=true\nUnit=zfs-mesh-scrub@%i.service\n\n[Install]\nWantedBy=timers.target\n"
+      "id": "scrub-run",
+      "type": "process",
+      "name": "zfs-mesh-scrub",
+      "artifact": "tools",
+      "run": [
+        "./zfs-tools",
+        "scrub-run"
+      ],
+      "schedule": "*/15 * * * *"
     },
     {
       "id": "distribution-weekly",
@@ -180,21 +177,6 @@
       "unit": "zfs-scrub-monthly@${setting:scrub-pool}.timer",
       "state": "stopped",
       "boot": "disabled"
-    },
-    {
-      "id": "scrub-schedule",
-      "type": "service",
-      "unit": "zfs-mesh-scrub-${setting:scrub-cadence}@${setting:scrub-pool}.timer",
-      "state": "running",
-      "boot": "enabled",
-      "restart-on": [
-        "scrub-service",
-        "scrub-weekly",
-        "scrub-monthly"
-      ],
-      "health": {
-        "kind": "unit"
-      }
     }
   ],
   "build": {