docs: call an authority Evidence deployment a gateway

[jeremi]

Why

cell was a private coinage of the authority-owned reset. It is defined
nowhere a reader can find, it collides with the local evidence-gateway
Caddy service in compose.yaml, and upstream Registry Stack never uses it
as a product term: its noun for the service that answers a requirement over
an authority's own source is Evidence gateway (165 occurrences in the
registry-stack docs).

What changed

Every surface a reader sees now says "authority Evidence gateway":

• Visitor Center copy: services.ts labels and blurbs, ProofStrip,
  SolmaraPreview, the anatomy page and its meta description
• purposes.ts enforcing-service string, plus the published
  docs/purposes.md catalogue
• docs/solmara.md, docs/hosted-deployment.md, both story docs, README.md
• the published DCAT/CPSV catalogue: the catalog description and the six
  data_services titles, with metadata/public/ regenerated

A dated changelog entry records the wording change and its limits.

What deliberately did not change

Identities are addresses, not descriptions, so renaming them would move
deployed routes or break a harvester:

• evidence/cells/, runtime/evidence-cells/, config/evidence/local/cells/
• *-authority-cells.solmara.registrystack.org hosts and
  /data/solmara-authority-cells/... volume paths
• metadata id: solmara-authority-cells
• the SOLMARA_<CELL>_EVIDENCE_PUBLIC_HOST variables
• evidence/scripts/build-cells.py and evidence/tests/test_cells.py

The evidence-gateway Caddy service in compose.yaml (local TLS front door,
11 references across 6 files) is untouched. The README entry point is now
described as "Local TLS front door" so the two no longer read as the same
thing, but whether that service should also be renamed is a separate call.

Verification

• just lint green (ruff, fiction, config secrets, image pins, runtime
  topology, release pin, signer keys, metadata publish --check,
  metadata-lint, portal pnpm check, home pnpm check)
• just test green: 302 Python tests, 143 portal, 62 home

TDD note

Updating the purposes.test.ts assertions alone stayed green, because the
fixture's five-column rows read enforcedBy from the table while the real
catalogue's three-column rows take the hardcoded branch. A new test covering
the three-column row went red first, then green.