Docs: build hash grammar, theme rollback, reference-identity change detection, option-list boundary rule

docs/architecture/04-routing-and-events.md

@@ -33,18 +33,22 @@ Zustand stores. Components never read `location.hash` or attach
 
 The hash is the serialized view. Astrolabe's forms:
 
-| State                       | Hash                           |
-| --------------------------- | ------------------------------ |
-| Default snippets view       | _(empty / absent)_             |
-| A selected snippet          | `#snippet-<id>`                |
-| Datasets manager (list)     | `#datasets`                    |
-| A specific dataset          | `#datasets/dataset-<id>`       |
-| New-dataset form            | `#datasets/new`                |
-| Chart Builder for a dataset | `#datasets/dataset-<id>/build` |
+| State                          | Hash                           |
+| ------------------------------ | ------------------------------ |
+| Default snippets view          | _(empty / absent)_             |
+| A selected snippet             | `#snippet-<id>`                |
+| Datasets manager (list)        | `#datasets`                    |
+| A specific dataset             | `#datasets/dataset-<id>`       |
+| New-dataset form               | `#datasets/new`                |
+| Chart Builder for a dataset    | `#datasets/dataset-<id>/build` |
+| Chart Builder, no dataset open | `#build`                       |
 
 Snippet `id` is an opaque string; dataset `id` is the numeric dataset id
 rendered as a decimal string. The hash is the **only** persisted view-routing
-state — there is no in-memory "current route" that can drift from it.
+state — there is no in-memory "current route" that can drift from it. `#build`
+serializes the builder's no-datasets state only: an un-targeted builder open
+picks a dataset itself when any exist, so the derived view immediately
+self-corrects to the `dataset-build` form.
 
 ### 1.2 The adapter: `infrastructure/url-hash.ts`
 
@@ -62,7 +66,8 @@ export type ViewState =
   | { kind: 'datasets' } // #datasets
   | { kind: 'dataset'; datasetId: number } // #datasets/dataset-<id>
   | { kind: 'dataset-new' } // #datasets/new
-  | { kind: 'dataset-build'; datasetId: number }; // .../build
+  | { kind: 'dataset-build'; datasetId: number } // .../build
+  | { kind: 'build' }; // #build — builder, no dataset loaded
 
 export function parseHash(rawHash: string): ViewState {
   const hash = rawHash.replace(/^#/, '');
@@ -71,6 +76,8 @@ export function parseHash(rawHash: string): ViewState {
   const snippet = /^snippet-(.+)$/.exec(hash);
   if (snippet) return { kind: 'snippet', snippetId: snippet[1] };
 
+  if (hash === 'build') return { kind: 'build' };
+
   const parts = hash.split('/').filter(Boolean);
   if (parts[0] === 'datasets') {
     if (parts.length === 1) return { kind: 'datasets' };
@@ -100,6 +107,8 @@ export function serializeHash(view: ViewState): string {
       return '#datasets/new';
     case 'dataset-build':
       return `#datasets/dataset-${view.datasetId}/build`;
+    case 'build':
+      return '#build';
   }
 }