Drawbridges (down, up, automatic), frost spell and obsidian over cooled lava

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/research.rs

@@ -34,7 +34,9 @@ pub const RESEARCH: &[(usize, f32)] = &[
     (25, 650.0),  // stone roller
     (37, 700.0),  // invisibility
     (19, 700.0),  // steel walls
+    (47, 700.0),  // drawbridge
     (28, 800.0),  // frost glow
+    (48, 800.0),  // frost spell
     (38, 850.0),  // plague
     (27, 900.0),  // rune mine
     (39, 950.0),  // quake