docs: add ubiquitous language, QFD design, and ADRs

docs/adr/0002-vue-flow-substrate.md

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+# Vue Flow is the editor substrate; the domain Model is the source of truth
+
+The editor is built on **Vue Flow** (`@vue-flow/core`) for pan/zoom, node
+dragging, handles, edge-drawing, and selection, with Stocks/Flows/Converters/
+Clouds as custom Vue node components. We rejected hand-rolling an SVG editor (too
+slow to the friction goals F1–F3) and heavier kits (AntV X6, mxGraph — not
+Vue-3-native).
+
+The constraint that makes this safe: the **domain `Model` is the single source of
+truth; the Vue Flow graph is a derived projection of it, never the reverse.** All
+writes go through store actions that mutate the `Model`; the Vue Flow view is
+re-derived. The future simulator reads the `Model` and has no knowledge of Vue
+Flow.
+
+## Consequences
+
+- A **projection layer** (Model → Vue Flow nodes/edges, and Vue Flow events →
+  store actions) must be built and kept thin. This is the price of the dependency.
+- Swapping the rendering substrate later means rewriting only the projection +
+  node/edge components, not the domain model or the simulator.
+- A reader wondering "why is there a sync/projection layer instead of editing Vue
+  Flow state directly?" should look here: it protects the simulate-later constraint.