ai: the gather order decides fleet names too, and not only ours

Lane BT found it while building the comparison: FtName is an id-attached label
minted by the same pass as the id, and its counter is per player and shared with
the server's fleets. In the original, a run that minted two client fleets named
an unrelated server fleet differently from a run that minted three. So a gather
that visits groups in a different order renames fleets the AI never touched.

src/game/ai/orders.h

@@ -194,6 +194,16 @@ struct ColonizeOrder {
 //      run states its own order for free.  An optional override that replays a captured order is
 //      what turns "equal modulo a relabelling" into "equal" -- three words per new fleet, the same
 //      cost class as pinning a seed.
+//   4. **The gather order also decides fleet NAMES, and not only ours.**  `FtName` is an
+//      id-attached label -- a Greek ordinal minted by the same pass as the id -- and its counter is
+//      PER PLAYER and SHARED WITH THE SERVER'S FLEETS.  In the original, the run that minted two
+//      client fleets named an unrelated node-0 fleet "Upsilon" and the run that minted three named
+//      that same fleet "Phi".  So a gather that visits groups in a different order renames fleets
+//      the AI never touched, across a boundary that has nothing to do with the AI.  Whatever we do
+//      here, the naming must come off the same ordered walk as the ids, or saves will differ in
+//      names while agreeing on everything else.  (Found by lane BT while building the comparison:
+//      "apply the relabelling to every id-valued leaf" would have produced two spurious name
+//      differences, because the name belongs to the slot and not to the group.)
 //
 // Evidence: sots-re `findings/control-flow/fleet-visit-order-mechanism.md` (the container and its
 // key, measured across three processes including a branch flip),