docs(docs): add frontmatter to all design docs

Standardized YAML frontmatter on all 40 docs/design/ files with title,
description, type, status, ticket, decision_refs, and author fields.
Covers specs, wireframes, voice cards, spatial layouts, and system
designs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/design/sound-indicator-visual.md

@@ -1,3 +1,15 @@
+---
+title: "Sound Indicator Visual Design"
+description: "Fog-edge pulse specification for directional sound indicators with range differentiation and suppression rules"
+type: design
+status: active
+ticket: "#317"
+decision_refs: [D-018, D-033, D-047, D-048, D-049, D-059, D-070]
+author: "Araminta"
+created: 2026-02-20
+updated: 2026-02-20
+---
+
 # Sound Indicator Visual Design — Fog-Edge Pulse Specification
 
 **Version:** v0.1 (Sprint 14)