feat: Complete 6 additional SDK implementations with fixes and examples

Go Async SDK (clients/go/async/):
- Fixed case-sensitive language detection bug (.R for R language)
- Created go.mod for module management
- Added comprehensive test suite
- Created 3 examples with expected output comments
- Added README with full documentation

Java Sync SDK (clients/java/sync/):
- RENAMED: Unsandbox.java -> Un.java (matches naming convention)
- Updated class name from Unsandbox to Un
- Created pom.xml for Maven build
- Added 6 examples (simple + SDK client versions)
- Created test suite with JUnit 5
- Added README with API documentation

JavaScript Async SDK (clients/javascript/async/):
- Fixed unused import
- Created package.json with ES module support
- Added 5 examples covering all async patterns
- Created 71 tests (all passing)
- Added comprehensive README

PHP Sync SDK (clients/php/sync/):
- RENAMED: Unsandbox.php -> un.php (matches naming convention)
- Created composer.json with PSR-4 autoloading
- Created phpunit.xml for testing
- Added 4 examples with expected output comments
- Created 54 tests across 4 test files
- Added README with full documentation

Ruby Sync SDK (clients/ruby/sync/):
- Created Gemfile and un.gemspec
- Created Rakefile with test task
- Updated examples to actually use the SDK
- Added 4 examples (hello_world, async_job, language_detection, snapshots)
- Created comprehensive test suite with 30+ tests
- Added README with documentation

Rust Sync SDK (clients/rust/sync/):
- Updated Cargo.toml with example declarations
- Created 4 examples (hello_world, fibonacci, multi_language, async_polling)
- Added comprehensive README with API reference
- All dependencies verified correct

All SDKs verified:
- HMAC-SHA256 authentication implemented
- 4-tier credential system (args > env > ~/.unsandbox > ./accounts.csv)
- Expected output comments for pipeline validation
- Proper error handling
- Language detection support

clients/javascript/async/examples/concurrent_execution.js

@@ -0,0 +1,76 @@
+#!/usr/bin/env node
+/**
+ * Concurrent Execution example for unsandbox JavaScript SDK
+ *
+ * Demonstrates running code in multiple languages concurrently.
+ * Shows the power of async/await with Promise.all() for parallel execution.
+ *
+ * To run:
+ *   export UNSANDBOX_PUBLIC_KEY="your-public-key"
+ *   export UNSANDBOX_SECRET_KEY="your-secret-key"
+ *   node concurrent_execution.js
+ *
+ * Expected output:
+ *   Starting concurrent execution in 4 languages...
+ *   [python] Output: Hello from Python!
+ *   [javascript] Output: Hello from JavaScript!
+ *   [go] Output: Hello from Go!
+ *   [ruby] Output: Hello from Ruby!
+ *   All executions completed in Xms
+ */
+
+import { executeCode, CredentialsError } from '../src/un_async.js';
+
+const LANGUAGE_CODE = {
+  python: 'print("Hello from Python!")',
+  javascript: 'console.log("Hello from JavaScript!");',
+  go: `package main
+import "fmt"
+func main() {
+    fmt.Println("Hello from Go!")
+}`,
+  ruby: 'puts "Hello from Ruby!"',
+};
+
+async function runCode(language, code) {
+  try {
+    const result = await executeCode(language, code);
+    const output = (result.stdout || '').trim();
+    console.log(`[${language}] Output: ${output}`);
+    return { language, output, success: true };
+  } catch (e) {
+    console.log(`[${language}] Error: ${e.message}`);
+    return { language, error: e.message, success: false };
+  }
+}
+
+async function main() {
+  try {
+    console.log('Starting concurrent execution in 4 languages...');
+    const startTime = Date.now();
+
+    // Execute all languages concurrently
+    const results = await Promise.all(
+      Object.entries(LANGUAGE_CODE).map(([lang, code]) => runCode(lang, code))
+    );
+
+    const elapsed = Date.now() - startTime;
+    console.log(`All executions completed in ${elapsed}ms`);
+
+    // Check for errors
+    const successCount = results.filter((r) => r.success).length;
+    console.log(`Success: ${successCount}/${results.length}`);
+
+    return successCount === results.length ? 0 : 1;
+  } catch (e) {
+    if (e instanceof CredentialsError) {
+      console.log(`Credentials error: ${e.message}`);
+    } else {
+      console.log(`Error: ${e.message}`);
+      console.error(e);
+    }
+    return 1;
+  }
+}
+
+main().then(process.exit);