fix(flights): don't treat OpenSky's position-less rows as a partial snapshot

OpenSky's state-vector spec allows null longitude/latitude, and every live
/states/all response carries such rows (about 50 of 7,800). The normaliser
rejects them, so admitRecords reported complete: false on effectively every
poll. While a snapshot is incomplete, FlightRecords.absence() retains any
aircraft received in the last five minutes, which meant the stale fade,
MISSING_POLL_LIMIT and the landed fast-cull never ran: vanished and landed
aircraft stayed on the globe at full opacity for up to five minutes.

admitRecords now takes an optional unpositionedId(row). A rejected row it
names is a contact without a position: its id is returned in retainIds and
the row does not count against completeness. OpenSky opts in for rows with an
ICAO24 address and strictly null longitude and latitude; the id comes from
the same helper the normaliser uses. Rows with no usable id, out-of-range or
half-null coordinates, truncated rows and positioned duplicates still make
the snapshot incomplete, and a nonempty feed with no positioned rows is
still malformed.

absence() retains exactly the retainIds aircraft under the existing
five-minute bound, except likely-landed ones, which keep the landed limit.
Everything else ages out normally. Military readsb admission does not opt
in and is unchanged.

src/layers/flights/records.js

@@ -258,10 +258,12 @@ export class FlightRecords {
     return { icao24, prevMeta, meta, groundFlipped, fixEpochMs };
   }
 
-  absence(id, { complete, likelyLanded }) {
-    // Partial admissions do not prove absence, but retention remains bounded.
+  absence(id, { complete, retainIds, likelyLanded }) {
+    // Partial admissions and position-less rows do not prove absence, but
+    // retention remains bounded; a likely-landed position-less contact
+    // still takes the landed limit.
     if (
-      !complete &&
+      (!complete || (retainIds?.has(id) && !likelyLanded)) &&
       Date.now() - (this.data.get(id)?.observedReceiptMs ?? 0) < 300000
     )
       return 'retain';