Add structure ignition risk checks

Docs/TechnicalDesignDocument.md

@@ -353,6 +353,14 @@ duration, wind speed, current weather, cleared-area maintenance, and the
 campfire's current risk ratio. Reaching the threshold marks the relevant
 ignition flag, while later spread rules decide how active fires propagate.
 
+Structure ignition risk uses the same server-authoritative campfire risk model.
+Open fires check nearby primitive shelters plus flammable wood/fiber resource
+nodes as MVP stand-ins for wood piles, flammable crafting stations, and future
+settlement objects. Contained fires skip this structure check; otherwise
+structure risk accumulates with distance-based fuel presence, burn duration,
+weather/wind, and the current fire-risk ratio before setting a replicated
+structure ignition flag.
+
 Campfires expose native extinguish logic through `AAgrarianCampfire::Extinguish`.
 Extinguishing clears remaining fuel, turns off replicated lit state, and reuses
 the same visual update path as natural fuel depletion.