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.

docs/roadmap.md

@@ -75,7 +75,9 @@ These three run in parallel and gate everything downstream:
     commissioning is mandatory for warranty; budget per the service rate
     card)
 17. Traceability software v1: intake, batch genealogy, press telemetry
-    capture (spec in [traceability.md](traceability.md))
+    capture (spec in [traceability.md](traceability.md), systems and build
+    order in [digital-factory.md](digital-factory.md); the network, servers
+    and edge collectors go in before the machinery arrives)
 18. Hire and train the launch team; safety program stands up with the
     equipment, not after it