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>

public/styles.css

@@ -120,3 +120,5 @@ a{color:inherit}.wrap{width:min(1160px,calc(100% - 32px));margin:auto}.nav{heigh
 img.qr-img{object-fit:contain}
 
 a.brand{text-decoration:none;color:inherit}
+
+.mtp-spill{font-size:10px;color:var(--teal);width:100%;margin-top:3px}