docs(wildlife): cite the documented iNaturalist UTFGrid endpoint and Acartia's key request

/grid/{z}/{x}/{y}.grid.json is documented in the v1 and v2 API docs as
Grid Tiles UTFGrid; only its per-cell records are unspecified, which is
what the schema guard covers. DATA_SOURCES.md now also notes that
Acartia's README asks API users to create an account and an API key,
although /sightings/current answers without one today.

docs/CURRENT-STATE.md

@@ -160,7 +160,8 @@ adjacent-zoom tiles publish before network waits; each arriving tile replaces
 its covered area, including healthy empty tiles. Grid tiles publish even while
 the independent observation request is pending or failed. Enable waits for one
 camera settle; camera acquisition uses moveEnd with a 600 ms debounce. The
-undocumented UTFGrid endpoint has a schema guard; see `KNOWN-ISSUES.md`.
+documented UTFGrid endpoint's unspecified cell records have a schema guard;
+see `KNOWN-ISSUES.md`.
 
 Ambient cards use a fixed 96 px image width and a short creator name credit,
 with a bounded frame cache. New cards fill free screen regions; visible cards

docs/KNOWN-ISSUES.md

@@ -10,12 +10,14 @@ For the roadmap and open backlog, see the repository issue tracker.
 
 ## Open
 
-### Wildlife grid is an undocumented iNaturalist endpoint
+### Wildlife grid relies on the iNaturalist UTFGrid cell schema
 
 Status: Open (guarded fallback)
 
-The operational `/v1/grid/{z}/{x}/{y}.grid.json` endpoint is not part of the
-published v1 Swagger contract. Its 64×64 UTFGrid shape, keys and cell records
+The `/v1/grid/{z}/{x}/{y}.grid.json` endpoint is documented in the v1 and v2
+API docs (“Grid Tiles UTFGrid”), but the documented response only defines the
+UTFGrid `grid`, `keys` and `data` members; the per-cell records are not
+specified. Its 64×64 UTFGrid shape, keys and cell records
 are validated in full at the allow-list proxy. A schema change or unavailable
 tile retains its last good cells for at most one hour; a cold unavailable grid
 uses sampled observations with the same badges and screen buckets, without