Teach spillover in the matrix views

Tree nodes now carry referrerId, so positions placed by spillover (their
referrer differs from their matrix parent) get a teal "spillover · ref
#N" tag on cards in both the member dashboard and admin matrix, plus a
legend entry. When a member is unqualified but has spillover positions
under them — the classic "why do I have 2 under me but no checkmark?"
state — a callout explains that spillover grows the matrix but only
members joining with their own ID count toward 2/2, and points them at
their share link.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

chain.js

@@ -412,7 +412,7 @@ function getSubtree(id, showDepth = 3) {
     const l = node(m.l, depth + 1), r = node(m.r, depth + 1);
     const n = {
       id: nid, tier: m.tier, level: m.level, levelName: levelName(m.level || 1),
-      directCount: m.directCount || 0, earnedPol: m.earnedPol || 0,
+      directCount: m.directCount || 0, earnedPol: m.earnedPol || 0, referrerId: m.referrerId || 0,
       downCount: 0, downPol: 0
     };
     for (const c of [l, r]) if (c) { n.downCount += 1 + c.downCount; n.downPol = +(n.downPol + c.earnedPol + c.downPol).toFixed(2); }