Place external source/exit pills near their connection targets

Replace count-sorted, group-centered rows with a spring-like placement:
pills order by their weighted median target x and settle by clamped
coordinate descent, minimizing weighted horizontal connection distance
while keeping the minimum pill spacing.

docs/analytics.md

@@ -438,8 +438,11 @@ map: each visit is attributed to `utm_campaign`, then `utm_source`, then the
 referer origin, then any other `utm_*` tag, so UTM-tagged visits are grouped
 under their campaign/source value rather than the referer domain. A UTM
 source node only links to its referer when every visit carrying that tag
-came from the same origin. External exits are full-size nodes in a matching
-row centered below the map, so the site itself stays in the middle), per-page view
+came from the same origin. Within the source and exit rows the pills are
+not sorted by count; each slides sideways toward the pages it connects to,
+minimizing the weighted horizontal connection distance while keeping a
+minimum pill spacing. External exits are full-size nodes in a matching
+row below the map, so the site itself stays in the middle), per-page view
 counts, the top transitions and the 50 most recent visit trails. Data is
 streamed live over `WebSocket /_api/ws/analytics`, which pushes the latest
 JSON snapshot on connect and again whenever the analytics file is updated