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);