fix(tools): list a split submarine cable once in find_submarine_cables

TeleGeography stores 17 cables (2Africa, Echo, Hawaiki...) as several
features under one id, and the tool made a row per feature, so "Echo"
read "3 submarine cables" and area counts ran high. Rows are now unique
by cable id, and a cable still matches when any of its parts does.

docs/CURRENT-STATE.md

@@ -179,6 +179,9 @@ catalog, loaded on first use. See [tools and the MCP server](TOOLS.md).
 Aircraft altitude-band queries require a reported barometric or geometric altitude
 for each match, so a grounded contact that reports none is left out of a band too.
 Queries without an altitude band retain unknown-altitude contacts.
+`find_submarine_cables` lists and counts each cable once: TeleGeography splits
+a few cables (2Africa, Echo) into several features under one id, and such a
+cable matches when any of its parts crosses the area.
 
 Catalog calls check their cancellation signal immediately before executing a
 tool. Already-cancelled queries do not start; cancellation after execution starts