Add CRAN platform support and documentation

Added CRAN (R package repository) support to the platform configuration and updated the README with usage instructions and examples for CRAN. Introduced tests for CRAN path transformation and configuration in a new test/cran.test.js file.

test/cran.test.js

@@ -0,0 +1,53 @@
+import { describe, expect, it } from 'vitest';
+import { PLATFORMS, transformPath } from '../src/config/platforms.js';
+
+describe('CRAN Platform Configuration', () => {
+  it('should have CRAN platform configured', () => {
+    expect(PLATFORMS.cran).toBe('https://cran.r-project.org');
+  });
+
+  it('should transform CRAN paths correctly', () => {
+    const testCases = [
+      {
+        input: '/cran/src/contrib/ggplot2_3.4.4.tar.gz',
+        expected: '/src/contrib/ggplot2_3.4.4.tar.gz',
+        description: 'package source file'
+      },
+      {
+        input: '/cran/web/packages/dplyr/DESCRIPTION',
+        expected: '/web/packages/dplyr/DESCRIPTION',
+        description: 'package description file'
+      },
+      {
+        input: '/cran/bin/windows/contrib/4.3/ggplot2_3.4.4.zip',
+        expected: '/bin/windows/contrib/4.3/ggplot2_3.4.4.zip',
+        description: 'Windows binary package'
+      },
+      {
+        input: '/cran/bin/macosx/big-sur-arm64/contrib/4.3/ggplot2_3.4.4.tgz',
+        expected: '/bin/macosx/big-sur-arm64/contrib/4.3/ggplot2_3.4.4.tgz',
+        description: 'macOS binary package'
+      },
+      {
+        input: '/cran/web/packages/packages.rds',
+        expected: '/web/packages/packages.rds',
+        description: 'package index file'
+      }
+    ];
+
+    testCases.forEach(({ input, expected, description }) => {
+      const result = transformPath(input, 'cran');
+      expect(result).toBe(expected, `Failed for ${description}: ${input}`);
+    });
+  });
+
+  it('should handle root path correctly', () => {
+    const result = transformPath('/cran/', 'cran');
+    expect(result).toBe('/');
+  });
+
+  it('should handle paths without platform prefix', () => {
+    const result = transformPath('/src/contrib/ggplot2_3.4.4.tar.gz', 'cran');
+    expect(result).toBe('/src/contrib/ggplot2_3.4.4.tar.gz');
+  });
+});