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.

server/src/routes/links.ts

@@ -1,7 +1,7 @@
 import type { FastifyInstance } from 'fastify';
 import { z } from 'zod';
-import { haversineMeters } from '../services/geo.js';
-import type { Claim, Place, User } from '../types.js';
+import { haversineMeters, segmentsIntersect } from '../services/geo.js';
+import type { Claim, LinkRow, Place, User } from '../types.js';
 
 const linkBody = z.object({
   fromPlaceId: z.number().int(),
@@ -23,6 +23,7 @@ export default async function linksRoutes(app: FastifyInstance) {
   const getExistingLink = db.prepare(`
     SELECT * FROM links WHERE (from_place_id = ? AND to_place_id = ?) OR (from_place_id = ? AND to_place_id = ?)
   `);
+  const getAllLinks = db.prepare('SELECT * FROM links');
   const insertLink = db.prepare(`
     INSERT INTO links (from_place_id, to_place_id, team, created_by_pubkey, created_at)
     VALUES (?, ?, ?, ?, ?)
@@ -62,6 +63,25 @@ export default async function linksRoutes(app: FastifyInstance) {
       return reply.code(409).send({ error: 'these places are already linked' });
     }
 
+    // Links are physical XM beams in the sky in Ingress — a new link can't cross
+    // any existing link, from either team. Links sharing an endpoint with the new
+    // one aren't "crossing" (that's just a portal with multiple links fanning out).
+    for (const existing of getAllLinks.all() as LinkRow[]) {
+      if (
+        existing.from_place_id === fromPlaceId ||
+        existing.from_place_id === toPlaceId ||
+        existing.to_place_id === fromPlaceId ||
+        existing.to_place_id === toPlaceId
+      ) {
+        continue;
+      }
+      const existingFrom = getPlace.get(existing.from_place_id) as Place;
+      const existingTo = getPlace.get(existing.to_place_id) as Place;
+      if (segmentsIntersect(fromPlace, toPlace, existingFrom, existingTo)) {
+        return reply.code(409).send({ error: `would cross an existing link (id ${existing.id})` });
+      }
+    }
+
     const result = insertLink.run(fromPlaceId, toPlaceId, user.team, req.userPubkey, nowSecs());
     return { id: result.lastInsertRowid, fromPlaceId, toPlaceId, team: user.team };
   });