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/jest.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  testEnvironment: 'node',
+  testMatch: ['**/*.test.js'],
+  collectCoverageFrom: ['turn.js'],
+};