Transition map beads: connector-colored elongated pulses

Beads now take their connector's color — accent for internal links,
plain text color for external ones (matching the tconn-exit styling) —
and render as an elongated pulse along the direction of travel instead
of a plain dot: a rounded capsule with a gradient-faded tail, rotated
to the edge's angle (cached per flow). Flow objects carry an 'external'
tag from buildFlows; the gradients live in the svg defs with stop
colors resolved from CSS so theme variables apply.

docs/analytics.md

@@ -465,7 +465,10 @@ flow along each edge, persisting across data reloads (emitters are keyed
 per edge direction and beads tracked by progress, so an unrelated count
 change never reshuffles them), emitted at a rate linearly proportional
 to the directional count with no in-flight limit, opposing directions
-offset onto parallel lanes. External sources and exits whose connectors are
+offset onto parallel lanes; each bead renders as an elongated pulse
+(gradient-faded tail) along its direction of travel, colored like its
+connector — accent for internal edges, plain text color for external
+ones. External sources and exits whose connectors are
 all culled by the width threshold are dropped from their rows themselves
 (the site's own page nodes always stay, connected or not). External sources show as a node row above the
 map: each visit is attributed to `utm_campaign`, then `utm_source`, then the