feat(fp): brief v3 — besluit-driven guided drafting

Compose the herregistratie letter from the besluit instead of a library hunt:
the behandelaar picks positief/negatief (+ reden-checkboxes for a negatief) and
the kern's standaardteksten follow the selection live.

Front-end (this increment):
- Kern is recomposed reactively from the besluit selection (new BesluitSelected
  machine msg + composeKern); the "Genereer conceptbrief" button is gone. The
  drafter's free text is preserved across a selection change.
- The editor shows only the editable sections; the locked aanhef/slot render in
  the preview, not the authoring surface. Slot is a case-type template section
  (per templateId), documented as such.
- The panel re-seeds from the letter via inferSelection() — the besluit + redenen
  are read back off the kern's passage blocks, so the selection survives reload
  with no new wire fields (derive, don't store).
- letter-section drops the now-redundant per-section passage picker (besluit owns
  standaardteksten); keeps free-text + block edit/move/remove.

Fix: app-checkbox now falls back to a unique per-instance id when checkboxId is
omitted. The CIBG styled checkbox routes clicks through the label, so the shared
id="undefined" made every reason label toggle the first input — the second
checkbox could never be checked. Verified live (Playwright): each reason toggles
independently.

Backend/seam (brief v3 WIP): besluit/reason passage tags on the wire + seed,
carried through the adapter parse boundary.

Specs updated (besluit, brief.machine) and the affected stories re-pointed at the
new API. FE lint + build + 253 vitest specs green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/BriefEndpointTests.cs

@@ -62,6 +62,16 @@ public class BriefEndpointTests(TestWebApplicationFactory factory) : IClassFixtu
     // global passages + the arts-scoped one; no other-beroep passages leak in.
     Assert.Contains(view!.AvailablePassages, p => p.PassageId == "p-kern-arts");
     Assert.All(view.AvailablePassages, p => Assert.True(p.Scope == "global" || p.Beroep == "arts"));
+
+    // Guided-drafting tags (WP-brief-v3): positief + negatief + reason-specific negatief.
+    Assert.Contains(view.AvailablePassages, p => p.Besluit == "positief");
+    Assert.Contains(view.AvailablePassages, p => p.Besluit == "negatief" && p.Reason == "onvoldoende_scholing");
+
+    // Case context is joined onto the screen DTO for the behandel scherm header.
+    Assert.Equal("19012345601", view.CaseContext.BigNummer);
+    Assert.Equal("arts", view.CaseContext.Beroep);
+    Assert.False(string.IsNullOrWhiteSpace(view.CaseContext.ZorgverlenerNaam));
+    Assert.False(string.IsNullOrWhiteSpace(view.CaseContext.AanvraagReferentie));
   }
 
   [Fact]