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/sources/live/contract.js

@@ -53,17 +53,30 @@ export function coordinates(latitude, longitude) {
   );
 }
 
-/** Admit a snapshot atomically; an all-invalid nonempty feed is unavailable. */
-export function admitRecords(rows, normalize, label) {
+/**
+ * Admit a snapshot atomically; an all-invalid nonempty feed is unavailable.
+ * `unpositionedId(row)` names a rejected row that is a valid contact without
+ * a position: it is returned in `retainIds` and does not make the snapshot
+ * incomplete.
+ */
+export function admitRecords(rows, normalize, label, unpositionedId) {
   if (!Array.isArray(rows))
     throw new LiveSourceError('malformed', `Malformed ${label} response`);
   const records = [];
   const ids = new Set();
+  const retainIds = new Set();
+  let unpositioned = 0;
   for (const row of rows) {
     const record = normalize(row);
     if (record && !ids.has(record.id)) {
       records.push(record);
       ids.add(record.id);
+    } else if (!record) {
+      const id = unpositionedId?.(row);
+      if (id) {
+        retainIds.add(id);
+        unpositioned += 1;
+      }
     }
   }
   if (rows.length && !records.length) {
@@ -71,7 +84,8 @@ export function admitRecords(rows, normalize, label) {
   }
   return {
     records,
-    complete: records.length === rows.length,
+    retainIds,
+    complete: records.length + unpositioned === rows.length,
     rejectedCount: rows.length - records.length,
   };
 }