M1: shared turn logic + characterization tests (39 green)

- npm workspaces: shared/, server/
- shared/turn.js: port turn logic verbatim from App.js (bugs preserved)
- 39 characterization tests lock current behavior
- gitignore: sqlite data, logs

.gitignore

@@ -1,4 +1,3 @@
-# .gitignore
 node_modules
 build
 dist
@@ -6,4 +5,9 @@ dist
 .env.local
 .env.development.local
 .env.test.local
-.env.production.local
+.env.production.local
+*.log
+data/*.sqlite
+data/*.sqlite-*
+server/data/*.sqlite
+server/data/*.sqlite-*