Initial scaffold: spec, architecture playbook, and M0 skeleton

src/core/theme.ts

@@ -0,0 +1,10 @@
+/**
+ * Theme identity — portable core. No browser APIs, no React.
+ *
+ * `UiTheme` is the app-wide theme token. It lives in `src/core/` (not in a
+ * store) because pure core logic needs it too: the Vega chart-config mapping
+ * (`chartConfigFor`, see docs/architecture/05) keys off the same union, and core
+ * must never import from `src/app/`. The store and the document `data-theme`
+ * mirror this value; this is its single definition.
+ */
+export type UiTheme = 'light' | 'experimental';