Two user-visible changes from the cache and ClickHouse fixes in
memgraph/zero#71, plus the changelog entry for that work.

Caching now needs every cached label to declare `attributes`. The cache
copies only the columns the mapping names, so a label declaring none would
be cached id-only and every property read back would be NULL; MemGQL
declines to cache rather than serve those rows. This is the one thing that
tripped an external reporter whose mapping was otherwise correct, so it is
documented both where caching is set up and next to the existing routing
note in the schema-file reference, which already covers the parallel
"declared attributes required" rule.

`metaFields.id` is now exposed as a property on vertices and edges, so
`RETURN n` includes it and `n.<id_column>` reads it by name.

Also replaces the note claiming the cache is validated only against
Iceberg: it now runs in CI against six sources, and graph-native sources
cannot be cached at all.

Rewrites the note added earlier in this branch. MemGQL no longer refuses
to cache a label that declares no attributes — it caches its ids and
topology, which is what traversals and graph algorithms run on, and treats
a read of an undeclared property as a cache miss served from the source.
Same rule routing applies, so the mapping format gains no obligation.

Also documents the new `cached_properties` column on SHOW GRAPH CACHES,
and corrects the claim that every `USE`-split part engages the cache
independently: UNION composites dispatch as a whole and never consult it
(memgraph/zero#75).

Catches the changelog up with two changes it was missing: SHOW GRAPH
CACHES now reports `hits` / `misses` as well as `cached_properties`, and
a warm table is no longer re-scanned from the source when a newly-touched
edge type pulls it in as an endpoint.

Also rewrites the cache-fidelity entry. It described the defect as "cached
nodes carried only their id", which now reads as contradicting the
behaviour change above it — caching a label id-only is supported. The
defect was answering property reads out of a fragment that never held
them.