feat: add CNAP survivor pension source record

[HirenGajjar]

Closes #18

Adds data/sources/lu/cnap-survivor-pension.yaml — a source record for the official CNAP survivor pension page.

---

Source details

• URL: https://cnap.public.lu/en/pensions/pension-survie.html
• Title: Survival pension / Pension de survie
• Institution: CNAP (Caisse nationale d'assurance pension)
• Languages: EN, FR, DE (all three versions available on cnap.public.lu)
• source_type: institutional_page — CNAP is a specific institution on .public.lu, distinct from the general Guichet government portal
• verification_status: discovered as required by the issue

Notes

• institution_ids is empty — no CNAP institution record exists yet in data/institutions/lu/. A separate institution record for CNAP should be created to link this source properly.
• Page covers survivor pension eligibility, qualifying periods, application process, and payment arrangements for surviving spouses, partners, and orphans.
• Followed guichet-death-life-event.yaml as template per issue guidance.

Verified

• python3 scripts/validate.py — passes (11 data files)
• Trailing newline present
• 1 file created, 28 insertions

[TommiLindfors]

Merged! Great work @HirenGajjar — this is a key source for the cross-border pension track. The note about the missing CNAP institution record is exactly the kind of forward-thinking that helps with #40. Speaking of which, the cross-border corridor scaffolding project is ready for you whenever you'd like to pick it up. It's the biggest issue we've created and would build directly on your source research expertise.