docs(discussions): add frontmatter to all discussion rounds

Standardized YAML frontmatter on all 26 docs/discussions/ files with
title, description, type, status, round number, and created date.
All marked as archived.

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

docs/discussions/round-13-engine-selection.md

@@ -1,3 +1,12 @@
+---
+title: "Round 13: Engine Selection"
+description: "Selected Godot client + Rust/bevy_ecs simulation server architecture; decided D-020, rejected R-004 through R-010"
+type: discussion
+status: archived
+round: 13
+created: 2026-02-10
+---
+
 # ROUND 13: "Engine selection"
 
 **Team Leader (Jeroen):** Present the full landscape, all of you.