feat(vessels): key vessels by their source's stable id, and share a selected vessel

AIS Vessels are keyed by the id their live source gives, with the MMSI as an
attribute: an MMSI can be shared, spoofed or reused, so two vessels with one
MMSI are now two vessels. Looking a vessel up or selecting it by MMSI still
finds it.

A share link made while a vessel is selected opens that vessel. A UUID vessel
id travels as 16 bytes in a new thing field (13). A shared thing that is not
reporting now says so in a sentence per kind, with its id (ICAO, NORAD, MMSI
or vessel UUID) on a second line.

changelog.d/0-contacts-follow-world-clock.md

@@ -3,5 +3,34 @@
   (`source.timeline`, read by `readSnapshotTimeline` from
   `gods-eye-view/sources/live`). Away from live they show the snapshot at the
   clock's time and move on with it as it plays; back at live they poll as
-  before. A source with no timeline keeps them live, unchanged (Sameh Khamis,
+  before. A source's `timeline.subscribe` may also tell of a better answer
+  for a time it has answered; paused, the layer loads the time shown again.
+  A source with no timeline keeps them live, unchanged (Sameh Khamis,
   @samehkhamis, #PR).
+- AIS Vessels are keyed by the stable id their live source gives (`id` on a
+  vessel record), not by MMSI: vessels can share an MMSI, through spoofing,
+  reused numbers or defaults such as 0, and a number can move to another
+  hull. Two vessels sharing an MMSI are now two vessels, each selected,
+  followed and carded on its own. The MMSI is an attribute (`mmsi`), shown
+  when known and `--` otherwise, up to ten digits; looking a vessel up or
+  selecting it by MMSI (an older link, say) still finds a vessel with it, and
+  a vessel with no name is no longer named by its id. `vesselSnapshot` takes
+  `idFor(row)` for a source whose rows carry an id of their own. Tool and voice results carry the vessel's id beside its MMSI
+  (`id`, or `vesselId` where `id` is a display label), and `get_vessel_track`
+  also takes an `id`. AISStream has only MMSIs, so its vessels are keyed by
+  MMSI and nothing changes there (Sameh Khamis, @samehkhamis, #PR).
+- A share link made while a vessel is selected (and the camera follows
+  nothing) now opens that vessel: the link names it, opens with the camera
+  where it was, looking at the vessel, and selects the vessel once the feed
+  has it, at the link's world time when it has one. A vessel id that is a
+  source's stable id, such as a UUID, travels as its 16 bytes in a new thing
+  field (13), so the link stays short (62 characters where a followed
+  aircraft's is 44). MMSI and other ids, and links already shared, are
+  unchanged; an app that predates the field opens such a link at the same
+  camera, without selecting the vessel. A camera that no follow offset can
+  carry (below the vessel's horizon, say) still makes a place view.
+  A shared flight, military flight, vessel or satellite that is not reporting
+  now says so in a sentence ("The shared vessel isn't reporting now — showing
+  where it was at 14:05"), with its id on a second line under it (ICAO,
+  NORAD, MMSI or vessel UUID) instead of at the head of the sentence (Sameh
+  Khamis, @samehkhamis, #PR).