feat: add product concept subtitle to radars

A Radar gains an optional Concept — a short subtitle naming the product
and its typical performances, rendered under the title and wrapping to a
balanced second line when long. Old radars default to an empty Concept
on load.

CONTEXT.md

@@ -24,6 +24,11 @@ A single integer in **{0, 1, 2, 3, 4, 5}** assigned to one (Criterion, Profile)
 
 - **Aliases to avoid:** *note* (French calque — use *Score*), *rating* (acceptable in user-facing copy, but *Score* is canonical in code).
 
+### Concept
+A short free-text subtitle on a Radar that names the product and helps the reader identify its typical performances at a glance. Optional — a Radar may have an empty Concept. Rendered under the Radar name (the title) on the chart, wrapping to a second line when long.
+
+- **Aliases to avoid:** *subtitle* (that's the visual role, not the domain term), *description* (too generic — a Concept is a tight positioning phrase, not prose), *tagline* (acceptable in user-facing copy, but *Concept* is canonical in code).
+
 ## Conventions
 
 ### Higher is better