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/extras/voice.json

@@ -0,0 +1,13 @@
+{
+  "tone": "neutral",
+  "pronoun": "third-person",
+  "headingStyle": "unknown",
+  "headingLengthClass": "tight",
+  "ctaVerbs": [],
+  "buttonPatterns": [],
+  "sampleHeadings": [],
+  "stats": {
+    "buttons": 0,
+    "headings": 0
+  }
+}