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/swagger.json

@@ -1398,6 +1398,9 @@
           },
           "orgTemplate": {
             "$ref": "#/components/schemas/OrgTemplateDto"
+          },
+          "caseContext": {
+            "$ref": "#/components/schemas/CaseContextDto"
           }
         },
         "additionalProperties": false
@@ -1414,6 +1417,28 @@
         },
         "additionalProperties": false
       },
+      "CaseContextDto": {
+        "type": "object",
+        "properties": {
+          "zorgverlenerNaam": {
+            "type": "string",
+            "nullable": true
+          },
+          "bigNummer": {
+            "type": "string",
+            "nullable": true
+          },
+          "beroep": {
+            "type": "string",
+            "nullable": true
+          },
+          "aanvraagReferentie": {
+            "type": "string",
+            "nullable": true
+          }
+        },
+        "additionalProperties": false
+      },
       "ChangeRequestRequest": {
         "type": "object",
         "properties": {
@@ -1723,6 +1748,14 @@
           },
           "isDefault": {
             "type": "boolean"
+          },
+          "besluit": {
+            "type": "string",
+            "nullable": true
+          },
+          "reason": {
+            "type": "string",
+            "nullable": true
           }
         },
         "additionalProperties": false