docs: add ubiquitous language, QFD design, and ADRs

docs/adr/0003-flow-as-node-materialised-clouds.md

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+# A Flow is a node; Source/Sink clouds are materialised nodes
+
+In the domain `Model`, a **Flow** is a *node* carrying `source` and `target`
+references (each to a Stock or Cloud), not an edge. The pipe you see is *rendered*
+from those references. The **Source/Sink** boundary is a **materialised Cloud
+node**, so every Flow connects node→node uniformly — no `null` ends.
+
+We rejected modelling a Flow as an edge (it breaks "an Information Link targets a
+Flow," since edges can't be edge endpoints, and it muddies the simulator) and
+rejected `null`-ended flows (special-casing in every graph walk and in rendering).
+
+## Consequences
+
+- The only stored edge type is the **Information Link**. Flows and Clouds are
+  nodes. A reader seeing a "node that renders as a pipe" and "phantom cloud nodes"
+  should look here.
+- **Cloud lifecycle** must be auto-managed: spawn a Cloud when a Flow is left
+  open-ended; remove it when the Flow attaches to a Stock; never leave orphans.
+- Loop detection and the future simulator both walk a uniform node graph
+  (info-links + flow→stock edges), with no boundary special cases.