Add designlang public-shell pack; tighten auth capture checks

Ran authenticated extract against coulomb.social; session metadata
matched anonymous Bubble defaults, so treat docs/design-extract as a
public shell baseline (PROVENANCE.md). Capture script now refuses to
overwrite storage-state.json with thin anonymous sessions.

docs/design-extract/prompts/v0.txt

@@ -0,0 +1,20 @@
+Build a landing page with this exact visual language.
+COLORS:
+  #000000
+  #171717
+  #d08728
+  #ffffff
+  #616161
+  #e5e5e5
+FONTS: [object Object], [object Object], [object Object], [object Object]
+SPACING: 2, 32, 68
+RADIUS: 4, 8
+SHADOWS: rgb(97, 97, 97) 2px 2px 8px 0px
+MATERIAL LANGUAGE: flat
+VOICE: Tone: neutral · Headings: unknown
+SECTIONS (in order):
+- hero
+- features
+- cta
+- footer
+Use Tailwind. Match these tokens exactly. Keep the material language consistent.