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/src/BigRegister.Api/Contracts/Dtos.cs

@@ -137,9 +137,13 @@ public sealed record BriefStatusDto(
     string? RejectedBy = null, string? RejectedAt = null, string? Comments = null,
     string? SentAt = null);
 
+// Besluit/Reason tag the passage for guided drafting (WP-brief-v3): the behandelaar
+// picks the besluit + reden and the FE filters this library to the matching passages.
+// null besluit = relevant to any besluit; null reason = not reason-specific.
 public sealed record LibraryPassageDto(
     string PassageId, string Scope, string SectionKey, string Label,
-    RichTextBlockDto Content, int Version, string? Beroep = null, bool IsDefault = false);
+    RichTextBlockDto Content, int Version, string? Beroep = null, bool IsDefault = false,
+    string? Besluit = null, string? Reason = null);
 
 public sealed record BriefDto(
     string BriefId, string Beroep, string TemplateId,
@@ -154,9 +158,14 @@ public sealed record BriefDecisionsDto(bool CanEdit, bool CanApprove, bool CanRe
 // The brief's screen DTO also carries the org template it renders with (WP-23):
 // the sub-org's current PUBLISHED version — or, once sent, the version pinned at
 // send time (sent letters are immutable; a republish never re-renders them).
+// The case this letter is about — the zorgverlener + aanvraag the behandelaar is
+// handling. Server-joined onto the brief's screen DTO so brief/ stays a shared-only
+// leaf context (no cross-context import of registratie).
+public sealed record CaseContextDto(string ZorgverlenerNaam, string BigNummer, string Beroep, string AanvraagReferentie);
+
 public sealed record BriefViewDto(
     BriefDto Brief, IReadOnlyList<LibraryPassageDto> AvailablePassages, BriefDecisionsDto Decisions,
-    OrgTemplateDto OrgTemplate);
+    OrgTemplateDto OrgTemplate, CaseContextDto CaseContext);
 
 public sealed record SaveBriefRequest(IReadOnlyList<LetterSectionDto> Sections);
 public sealed record RejectBriefRequest(string Comments);

backend/src/BigRegister.Api/Data/BriefStore.cs

@@ -176,6 +176,9 @@ public static class BriefSeed
 {
   public const string TemplateId = "besluit-arts";
   public const string Beroep = "arts";
+  // Demo case reference shown in the behandel scherm header (no real aanvraag linkage
+  // in this POC — one demo case).
+  public const string AanvraagReferentie = "HER-2026-000842";
 
   private static RichTextNodeDto T(string t) => new("text", Text: t);
   private static RichTextNodeDto P(string key) => new("placeholder", Key: key);
@@ -226,12 +229,22 @@ public static class BriefSeed
         {
             new("p-aanhef-1", "global", "aanhef", "Standaard aanhef",
                 Block(T("Geachte heer/mevrouw "), P("naam_zorgverlener"), T(",")), 1),
-            // The "standaardbrief" kern starter set (IsDefault): one button drops these
-            // in when the kern is still empty (WP-27).
-            new("p-kern-1", "global", "kern", "Beoordeling ontvangen",
-                Block(T("Op "), P("datum"), T(" hebben wij uw aanvraag beoordeeld.")), 1, IsDefault: true),
+            // Kern guidance set (WP-brief-v3). besluit=null → shown for any besluit (the
+            // shared intro); besluit=positief/negatief → inserted when that besluit is
+            // chosen; reason!=null → only when that reden is ticked. The FE filters this
+            // with passagesForBesluit — no server endpoint for guidance.
+            new("p-kern-intro", "global", "kern", "Beoordeling ontvangen",
+                Block(T("Op "), P("datum"), T(" hebben wij uw aanvraag beoordeeld.")), 1),
             new("p-kern-arts", "beroep", "kern", "Toelichting arts",
-                Block(T("Als arts met BIG-nummer "), P("big_nummer"), T(" delen wij u het volgende mee.")), 1, Beroep: "arts", IsDefault: true),
+                Block(T("Als arts met BIG-nummer "), P("big_nummer"), T(" delen wij u het volgende mee.")), 1, Beroep: "arts"),
+            new("p-kern-positief", "global", "kern", "Toewijzing",
+                Block(T("Uw aanvraag tot herregistratie is toegewezen. U blijft ingeschreven in het BIG-register.")), 1, Besluit: "positief"),
+            new("p-kern-negatief", "global", "kern", "Afwijzing",
+                Block(T("Uw aanvraag tot herregistratie is afgewezen.")), 1, Besluit: "negatief"),
+            new("p-kern-scholing", "global", "kern", "Onvoldoende scholing",
+                Block(T("U voldoet niet aan de eis van voldoende recente werkervaring en scholing.")), 1, Besluit: "negatief", Reason: "onvoldoende_scholing"),
+            new("p-kern-gegevens", "global", "kern", "Onjuiste gegevens",
+                Block(T("De door u aangeleverde gegevens zijn onjuist of onvolledig gebleken.")), 1, Besluit: "negatief", Reason: "onjuiste_gegevens"),
             new("p-slot-1", "global", "slot", "Standaard slot",
                 Block(T("Met vriendelijke groet,")), 1),
             // These reference a deprecated / not-fillable field so the linter's

backend/src/BigRegister.Api/Program.cs

@@ -450,7 +450,10 @@ BriefViewDto ToView(HttpContext ctx, BriefEntity e) => new(
     Authz.Decisions(Authz.ResolvePrincipal(ctx), e.Status.Tag, e.DrafterId),
     // Sent letters render with the version pinned at send; everything else follows
     // the sub-org's current published template (WP-23 immutability invariant).
-    OrgTemplateStore.TemplateForBrief(e.SubOrgId, e.Status.Tag == "sent" ? e.SentOrgTemplateVersion : null));
+    OrgTemplateStore.TemplateForBrief(e.SubOrgId, e.Status.Tag == "sent" ? e.SentOrgTemplateVersion : null),
+    // The case this letter is about — joined from the seeded zorgverlener so the
+    // behandel scherm can show whom/what it concerns without brief/ importing registratie.
+    new CaseContextDto(SeedData.Registration.Naam, SeedData.Registration.BigNummer, e.Beroep, BriefSeed.AanvraagReferentie));
 
 // Emit (decision flags, via ToView) and enforce (Forbidden/Conflict below) both run
 // through Authz — see BriefStore.Review and Authz.CanActOn — so they cannot drift.