Add multiplayer networking design

AGRARIAN_DEVELOPMENT_ROADMAP.md

@@ -567,13 +567,13 @@ Target deliverable: A small group can join a server, spawn into one biome, gathe
 
 ## 1.12 Basic Multiplayer
 
-- [ ] Confirm listen server vs dedicated server for MVP.
+- [x] Confirm listen server vs dedicated server for MVP. Decision: listen server is acceptable for quick internal testing, but dedicated server is the preferred closed-test target.
 - [ ] Create dedicated server build target if needed.
 - [ ] Add server travel flow.
-- [ ] Define server authority over streamed terrain tiles.
-- [ ] Define server response when a client requests a missing tile.
-- [ ] Add player join flow.
-- [ ] Add player spawn flow.
+- [x] Define server authority over streamed terrain tiles.
+- [x] Define server response when a client requests a missing tile.
+- [x] Add player join flow.
+- [x] Add player spawn flow.
 - [x] Add replicated player stats.
 - [x] Add replicated inventory.
 - [x] Add replicated world time.
@@ -1431,7 +1431,7 @@ Earliest incomplete foundation items:
 - [x] Confirm project opens cleanly from a fresh checkout.
 - [x] Create the core design document.
 - [x] Create the technical design document.
-- [ ] Create the multiplayer/networking design document.
+- [x] Create the multiplayer/networking design document.
 - [ ] Create the persistence design document.
 - [ ] Create the Earth-scale terrain/tile streaming design document.
 - [x] Launch near-term MVP map-tile serving cloud VM and prove Ground Zero tile lookup/download/cache flow.
@@ -1443,4 +1443,4 @@ Earliest incomplete foundation items:
 
 Immediate next item:
 
-- [ ] Create the multiplayer/networking design document.
+- [ ] Create the persistence design document.