Add digital factory systems plan (NixOS fleet as one Clan)

New docs/digital-factory.md: every system needed for environmental,
machine, QC and genealogy data to land in one database keyed to one
record identity. Standing decision: all services run on NixOS machines
declared and deployed as a single clan.lol inventory (mesh, backups,
host monitoring, secrets from clan services).

Covers machine list and roles, VLAN zoning, Telegraf/MQTT acquisition
source by source with the RFQ clause for each, environment sensor zones,
QC instrument integration paths, the factory application modules and
entities, alerting tiers, backup and retention, security, procurement
clauses, build order aligned with the roadmap phases, rough cost and
open questions.

README index and layout tree updated; roadmap item 17 and the
traceability spec link to the new document.

README.md

@@ -86,6 +86,13 @@ The detail lives in dedicated documents:
 - [Process logging and traceability](docs/traceability.md): end-to-end
   per-order traceability for customers, batch genealogy from audio file to
   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
 - [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
@@ -141,6 +148,7 @@ docs/
 ├── suppliers/             Supplier contacts, vendor landscape, supplier map
 ├── biomass.md             Biomass fuel strategy and the 60 rai coppice idea
 ├── boi.md                 BOI promotion strategy and requirements
+├── digital-factory.md     Data platform, NixOS/Clan fleet, acquisition, app
 ├── facility-security.md   Security, HVAC, fire and life safety, compliance
 ├── process-flow.md        Audio file to delivered record, with all checks
 ├── production-timeline.md Order turnaround, stage by stage