docs: cross-link design docs with README

Add a Design & docs section to the README linking CONTEXT, DESIGN, and the
three ADRs, plus a backlink to the README from each of those documents.

docs/adr/0001-detected-feedback-loops.md

@@ -1,5 +1,7 @@
 # Feedback loops are detected from a polarity-annotated graph, not stored
 
+_Part of [meadows](../../README.md) · see [DESIGN.md](../../DESIGN.md)._
+
 A **Feedback Loop** in a **Model** is a cycle in the wiring (Stock → Information
 Link(s)/Flow → same Stock). We chose to **derive** loops by detecting cycles and
 classifying each as **Reinforcing** (even number of `−` polarities) or

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

@@ -1,5 +1,7 @@
 # Vue Flow is the editor substrate; the domain Model is the source of truth
 
+_Part of [meadows](../../README.md) · see [DESIGN.md](../../DESIGN.md)._
+
 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

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

@@ -1,5 +1,7 @@
 # A Flow is a node; Source/Sink clouds are materialised nodes
 
+_Part of [meadows](../../README.md) · see [DESIGN.md](../../DESIGN.md)._
+
 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