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/traceability.md

@@ -58,4 +58,6 @@ For any order, the customer can follow the full chain:
   one
 
 This system should be specified alongside equipment procurement: machine
-data access is a purchasing requirement, not an afterthought.
+data access is a purchasing requirement, not an afterthought. The systems
+that implement this specification (machines, networks, collectors,
+database, application) are planned in [digital-factory.md](digital-factory.md).