Record the Making Vinyl feedback on traceability

Most people the team talked to in Haarlem were strongly interested in the
full-traceability approach. For Sibert it was one of the reasons to talk to
us at all; without it their representative would strongly advise against
the plan, and seeing buildfor.life's development and data work convinced
him the project comes from the right direction.

Exhibitor briefing gets an "After the show" section (feedback, what has
been filed since); traceability.md opens with the market signal; roadmap
item 1 notes it as supply-side evidence, with buyer prices still missing;
README project status updated (show attended, Payap lead).

docs/roadmap.md

@@ -17,7 +17,11 @@ These three run in parallel and gate everything downstream:
    recorded in the revenue model. The prices disagree with the model on
    per-title services and run size; the tour showed no plating and no
    HVAC anywhere in the incumbent's operation, which is the strongest
-   argument yet for pulling galvanics forward from Phase 5. Carry the
+   argument yet for pulling galvanics forward from Phase 5. Making Vinyl
+   Europe 2026 added supply-side evidence (strong interest in the
+   traceability approach, see the
+   [exhibitor briefing](suppliers/making-vinyl-europe-2026-exhibitors.md#after-the-show)),
+   but no buyer prices yet. Carry the
    open run-size question into these conversations. The
    [artist platform](artist-platform.md)'s price calculator, published
    early with indicative prices and an interest list but no payments,