Enforce Ingress-style link-crossing prevention

A new link can no longer cross any existing link from either team —
segmentsIntersect() does a standard orientation-based segment test,
and routes/links.ts excludes shared-endpoint pairs (fanning multiple
links out of the same portal is fine, that's not a crossing). 6 new
tests; 37 total passing.

README.md

@@ -44,6 +44,9 @@ Map's public API.
   links running through it.
 - A link can be created between any two places your team currently holds,
   as long as you're standing at the origin place — no maximum link distance.
+- A new link can't cross any existing link (either team) — same as Ingress.
+  Links sharing an endpoint don't count as crossing. See
+  `services/geo.ts#segmentsIntersect` and `routes/links.ts`.
 - Three mutually-linked, same-team places form a field (computed live, not
   stored) — see `server/src/services/fields.ts`.
 - Team choice is one-way once made.