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.

docs/CURRENT-STATE.md

@@ -825,7 +825,13 @@ addition to the locally accumulated trail, never a promise of complete coverage.
 
 Snapshot coverage, completeness and freshness are separate fields. A partially
 admitted aircraft snapshot retains absent contacts for up to five minutes before
-the usual missed-poll eviction. Unknown snapshot times remain unknown in stats. An invalid nonempty
+the usual missed-poll eviction. An OpenSky row with an ICAO24 address but null
+longitude and latitude is a contact without a position, not a malformed row: it
+leaves the snapshot complete and is listed in `retainIds`, so civil flights
+retain that aircraft under the same five-minute bound, unless it is likely
+landed, while other absent aircraft fade and leave on the missed-poll limits. A
+nonempty snapshot with no positioned rows is still malformed. Military readsb
+admission is unchanged. Unknown snapshot times remain unknown in stats. An invalid nonempty
 snapshot retains the previous display. Empty vessel refreshes retain the existing
 first-connect grace and warm-data behavior. Known source failures have bounded
 messages; arbitrary HTTP response bodies are not surfaced as diagnostics.