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

shared/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "@ttrpg/shared",
+  "version": "0.1.0",
+  "private": true,
+  "description": "Pure logic shared by client + server + tests. No I/O.",
+  "main": "index.js",
+  "scripts": {
+    "test": "jest"
+  },
+  "devDependencies": {
+    "jest": "^29.7.0"
+  }
+}