Digital factory plan: build on the existing cnx-network clan

Rewrite docs/digital-factory.md against the fleet that already exists at
git.b4l.co.th/B4L/cnx-network-clan instead of a plant-only clan:

- The plant is one more site: gw-lpn-1 from the cnx.router module, plant
  machines in the same inventory, reached over the existing ZeroTier mesh
  (no VPS, no moon, no port forwards).
- Fleet pieces reused as-is: control for host metrics, journals and host
  alerts; ns1/ns2 for the gateway wildcard cert; web01 as the public
  proxy for the customer portal; clan borgbackup with a plant server.
- Process time series move from TimescaleDB to a plant-local
  VictoriaMetrics (same modules and rules as control); PostgreSQL keeps
  everything with identity. Object storage dropped for ZFS datasets.
- VLAN table on the fleet addressing convention (siteId 3 assumed, ids
  above 40); OT segments as dual-homed islands behind the collectors;
  stations reach the app only through the gateway Caddy proxy.
- New section listing every file that changes in the clan repo, the
  modules/vinyl/* to write, and three router-module extensions (NTP
  serving, static VLANs, second WAN).
- Gaps the plant closes for the fleet: Alertmanager delivery, off-site
  backup destination, TSDB backup.
- Repository layout decided: infra in cnx-network-clan, app in
  cnx-factory as a flake input, this repo stays documents.

README bullet updated to match.

README.md

@@ -88,11 +88,12 @@ The detail lives in dedicated documents:
   shipment, and the requirements this sets for the factory software
 - [Digital factory systems plan](docs/digital-factory.md): every system
   that has to exist for environment, machine, QC and genealogy data to land
-  in one database keyed to one record identity: the NixOS machine fleet
-  managed as one Clan, networks and VLAN zoning, Telegraf/MQTT acquisition
-  source by source, QC instrument integration, the factory application,
-  alerting, backup, security, the data clauses for every RFQ, build order
-  and rough cost
+  in one database keyed to one record identity: the plant as one more site
+  in B4L's existing NixOS fleet (the cnx-network clan: mesh, DNS, fleet
+  monitoring, site gateway module, public proxy), what changes in that
+  repo, VLAN zoning, Telegraf/MQTT acquisition source by source, QC
+  instrument integration, the factory application, alerting, backup,
+  security, the data clauses for every RFQ, build order and rough cost
 - [Quality automation](docs/quality-automation.md): stamper metrology
   (Keyence-class confocal, profiler, XRF), layered pressed-record QC (100%
   optical inspection, per-cycle press telemetry, sampled automated audio