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/traceability.md

@@ -5,6 +5,13 @@ traceable end to end for the customer. This is a set requirement, and it is
 the core specification for the factory software this repository will
 eventually host.
 
+It is also the part of the plan the industry responds to. At Making Vinyl
+Europe 2026 most people the team talked to were strongly interested in it,
+and Sibert's representative named it as a reason to work with a newcomer
+he would otherwise have advised against building a plant; buildfor.life's
+development and data work settled the question for him (see the
+[exhibitor briefing](suppliers/making-vinyl-europe-2026-exhibitors.md#after-the-show)).
+
 ## What a customer can trace
 
 For any order, the customer can follow the full chain: