Chart Builder: open a snippet in the builder to edit it in place

docs/architecture/03-modal-system.md

@@ -284,6 +284,13 @@ export function toggleDatasets(): void {
 }
 ```
 
+> **Pre-hydrate variant — open without `init`.** When a modal must open onto state the
+> caller already loaded (the Chart Builder's _Open in builder_ edit flow — `openChartBuilderForEdit`
+> hydrates the builder from a snippet first), the opener sets the active modal, snapshots, and
+> syncs the URL **itself** and **skips the registry `init`** — running `init` would re-derive the
+> default state and clobber the hydration. Such an opener is the exception, not the rule: ordinary
+> opens go through `openModal` so `init` is the single place transient state is seeded.
+
 ### Change detection
 
 ```ts