feat: add Bureau de l'état civil institution, death certificate source, and link issuing authority (#38)

Adds Bureau de l'etat civil institution record, dedicated death certificate source record, and links issuing_authority_id in the death certificate document requirement. Multi-file, schema-compliant, source-cited. Excellent work by @HirenGajjar.

Author: HirenGajjar

data/document_requirements/lu/death-certificate.yaml

@@ -9,16 +9,17 @@ jurisdiction:
   country: LU
   region: null
   municipality: null
-issuing_authority_id: null
+issuing_authority_id: institution:lu:bureau-etat-civil
 accepted_forms:
   - certified_copy
   - copy
 source_ids:
   - source:lu:guichet:death-life-event
+  - source:lu:guichet:death-certificate
 verification_status: structured-from-source
 provenance:
   created_at: "2026-05-14"
   created_by: "clarvia-maintainers"
-  updated_at: "2026-05-23"
+  updated_at: "2026-05-25"
   updated_by: "HirenGajjar"
-notes: "Issuing authority is the civil registrar's office (Bureau de l'état civil) of the communal administration in the commune where the death occurred — institution record not yet in data/institutions/lu/, to be created. Accepted forms verified from Guichet: certified_copy (extracts issued as standard) and copy (additional copies available on request)."
+notes: "Issuing authority confirmed as institution:lu:bureau-etat-civil — the civil registrar's office of the communal administration in the commune where the death occurred. Institution and source records created in PR #37."

data/institutions/lu/bureau-etat-civil.yaml

@@ -0,0 +1,26 @@
+id: institution:lu:bureau-etat-civil
+object_type: Institution
+name: "Bureau de l'état civil (Civil Registrar's Office)"
+jurisdiction:
+  country: LU
+  region: null
+  municipality: null
+function: "Registers civil status events including births, marriages, and deaths. Receives death declarations and issues death certificates and related documents on behalf of the communal administration."
+official_site: "https://guichet.public.lu/en/citoyens/justice/certificats-copies-actes-casier-judicaire/certificats-copies-actes/acte-deces.html"
+contact_channels:
+  - type: website
+    value: "https://guichet.public.lu/en/citoyens/justice/certificats-copies-actes-casier-judicaire/certificats-copies-actes/acte-deces.html"
+    notes: "Guichet.lu page listing all communal administration contact details for death certificate requests."
+languages:
+  - fr
+  - de
+  - lb
+source_ids:
+  - source:lu:guichet:death-certificate
+verification_status: structured-from-source
+provenance:
+  created_at: "2026-05-25"
+  created_by: "HirenGajjar"
+  updated_at: null
+  updated_by: null
+notes: "The Bureau de l'état civil exists within every communal administration in Luxembourg. This record represents the institution type, not any specific commune's office. Only the commune where the death occurred can issue a death certificate."

data/sources/lu/guichet-death-certificate.yaml

@@ -0,0 +1,29 @@
+id: source:lu:guichet:death-certificate
+object_type: Source
+title: "Death certificate"
+title_fr: "Acte de décès"
+description: "Official Luxembourg government guidance on obtaining a death certificate, including the issuing authority, required supporting documents, accepted forms, and the information contained in the certificate."
+description_fr: "Guide officiel du gouvernement luxembourgeois sur l'obtention d'un acte de décès, notamment l'autorité émettrice, les pièces justificatives requises, les formes acceptées et les informations contenues dans le document."
+source_type: government_portal
+url: "https://guichet.public.lu/en/citoyens/justice/certificats-copies-actes-casier-judicaire/certificats-copies-actes/acte-deces.html"
+jurisdiction:
+  country: LU
+  region: null
+  municipality: null
+languages:
+  - en
+  - fr
+  - de
+institution_ids:
+  - institution:lu:guichet
+accessed_at: "2026-05-25"
+last_verified_at: null
+evidence_locator: null
+reuse_notes: "Official Luxembourg public-service portal source. Extract only source-backed administrative facts."
+verification_status: discovered
+provenance:
+  created_at: "2026-05-25"
+  created_by: "HirenGajjar"
+  updated_at: null
+  updated_by: null
+notes: "Specific Guichet page for the death certificate document. Distinct from the general death life event page. Used to verify the issuing authority and accepted forms in PR #34."