fix: properly organize shared test resources

- Move fixtures/ and unit/ to shared/ directory where they belong
- These test resources are used by both npm and native installations
- Update package.json test scripts to use new paths
- Fix relative import path in helpers.test.js
- All 46 npm-based tests now passing (7 unit + 5 integration + 34 npm)

Final test structure:
- native/     - Native installation tests only
- npm/        - npm package tests only
- shared/     - Shared utilities, fixtures, and unit tests
- integration/- Cross-installation integration tests
- edge-cases.sh/.ps1 - Master orchestrators

tests/shared/unit/helpers.test.js

@@ -1,7 +1,7 @@
 const assert = require('assert');
 const path = require('path');
 const os = require('os');
-const { expandPath, validateProfileName, isPathSafe } = require('../../bin/helpers');
+const { expandPath, validateProfileName, isPathSafe } = require('../../../bin/helpers');
 
 describe('helpers', () => {
   describe('expandPath', () => {