docs: update design spec and increase section gap to 80px

Update design spec with implementation notes covering recursive
compound nesting, edge z-ordering, ON_COMPLETION sections, drill-down
navigation, CSS transform zoom, and HTML overlay toolbar.

Increase SECTION_GAP to 80px for better visual separation between
completion and error handler sections.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ProcessDiagram/useDiagramData.ts

@@ -4,7 +4,7 @@ import type { DiagramNode, DiagramEdge } from '../../api/queries/diagrams';
 import type { DiagramSection } from './types';
 import { isErrorCompoundType, isCompletionCompoundType } from './node-colors';
 
-const SECTION_GAP = 40;
+const SECTION_GAP = 80;
 
 export function useDiagramData(
   application: string,