docs: establish simulation architecture gate

docs/README.md

@@ -0,0 +1,30 @@
+# Documentation map
+
+The project uses a small hierarchy so overlapping plans do not become competing
+sources of truth.
+
+1. [`PROJECT_CONTEXT.md`](PROJECT_CONTEXT.md) is the canonical description of
+   the vision, current implementation, target architecture, and active
+   constraints.
+2. [`LEARNING_ROADMAP.md`](LEARNING_ROADMAP.md) owns milestone order,
+   architecture gates, reusable-system exit tests, and intentionally deferred
+   work.
+3. [`BUILD_IN_PUBLIC_PLAN.md`](BUILD_IN_PUBLIC_PLAN.md) owns the scoped Jajce
+   visual slice. It must respect the architecture gates in the learning
+   roadmap.
+4. [`RESOURCE_NODE_MIGRATION.md`](RESOURCE_NODE_MIGRATION.md) is a focused
+   migration plan. Phases 1–4 are complete; its Phase 5 hands world placement to
+   the visual-slice plan.
+5. [`decisions/`](decisions/) contains durable architectural decisions,
+   including consequences and revisit conditions.
+
+When documents disagree:
+
+- code and tests describe current behavior;
+- the newest accepted decision record governs architecture;
+- `PROJECT_CONTEXT.md` governs product intent;
+- `LEARNING_ROADMAP.md` governs what should be built next;
+- focused plans govern only their stated scope.
+
+Update the smallest relevant set of documents after a decision or milestone.
+Do not copy full status tables into every plan.