refactor(shared): add BLOB_PRESENTER, unlock the blob-to-browser success paths (RB-28)

Three application-layer commands ended in raw DOM calls (URL.createObjectURL,
window.open, document.createElement('a').click(), URL.revokeObjectURL) as
their last statement. jsdom cannot assert a call that is also the end of the
function, so each command's success path stayed unassertable, and
StamdataStore.download()'s two-clause guard stayed permanently dark on its
true branch (TE-006).

Add BLOB_PRESENTER (libs/shared/src/application/blob-presenter.ts), an
InjectionToken mirroring SESSION_PORT's shape: an interface with open()/
download(), a real implementation preserving the existing open()-never-
revokes vs download()-always-revokes asymmetry, provided in root. Route
StamdataStore.download(), BriefStore.previewLetter(), and
OrgTemplateStore.proefbrief() through it.

Add specs with a recording fake presenter: StamdataStore.download()'s guard
(both clauses) and its success path, asserting toJson(...)'s exact output
reaches the file; BriefStore.previewLetter()'s existing success test now
goes through the seam instead of spying on window/URL directly; a new
org-template.store.spec.ts (none existed before) covers proefbrief()'s
success and failure paths.

Verified red without the fix by editing the download() filename to the
wrong extension, watching the success-path spec fail, then restoring it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

libs/beheer/src/application/stamdata.store.ts

@@ -18,6 +18,7 @@ import {
   reduce,
 } from '@beheer/domain/stamdata-editor.machine';
 import { StamdataAdapter } from '@beheer/infrastructure/stamdata.adapter';
+import { BLOB_PRESENTER } from '@shared/application/blob-presenter';
 
 type LoadedState = Extract<StamdataEditorState, { tag: 'loaded' }>;
 
@@ -30,6 +31,7 @@ type LoadedState = Extract<StamdataEditorState, { tag: 'loaded' }>;
 @Injectable({ providedIn: 'root' })
 export class StamdataStore {
   private adapter = inject(StamdataAdapter);
+  private blobPresenter = inject(BLOB_PRESENTER);
   private store = createStore<StamdataEditorState, StamdataEditorMsg>(initial, reduce);
 
   readonly model = this.store.model;
@@ -138,12 +140,7 @@ export class StamdataStore {
     const s = this.loaded();
     if (!s || !this.canDownload()) return;
     const blob = new Blob([toJson(s.table, s.rows)], { type: 'application/json' });
-    const url = URL.createObjectURL(blob);
-    const a = document.createElement('a');
-    a.href = url;
-    a.download = `${s.table.id}.json`;
-    a.click();
-    URL.revokeObjectURL(url);
+    this.blobPresenter.download(blob, `${s.table.id}.json`);
   }
 }