feat: add createIsolatedTerminal() test helper

Created a reusable test helper that ensures complete test isolation
by creating a fresh Ghostty WASM instance for each terminal.

Benefits:
- Single pattern for all tests: await createIsolatedTerminal()
- No manual Ghostty instance management in test files
- Clean slate guarantee - each test has isolated WASM memory
- Parallel-ready - no shared state between tests
- Simpler test code - just one function call

Updated all test files:
- lib/test-helpers.ts: New helper function
- lib/buffer.test.ts: Uses createIsolatedTerminal()
- lib/terminal.test.ts: Uses createIsolatedTerminal()
- lib/scrolling.test.ts: Uses createIsolatedTerminal()
- lib/selection-manager.test.ts: Uses createIsolatedTerminal()
- lib/input-handler.test.ts: Uses fresh Ghostty per test

All 283 tests pass with complete isolation.

Author: sreya

lib/buffer.test.ts

@@ -2,30 +2,23 @@
  * Buffer API tests
  *
  * Test Isolation Pattern:
- * Each test creates its own Ghostty WASM instance to ensure complete isolation.
- * This allows tests to run in parallel without shared state, and ensures a
- * clean slate for each test without worrying about previous test side effects.
+ * Uses createIsolatedTerminal() to ensure each test gets its own WASM instance.
  */
 
 import { afterEach, beforeEach, describe, expect, test } from 'bun:test';
-import { Ghostty } from './ghostty';
-import { Terminal } from './terminal';
+import type { Terminal } from './terminal';
+import { createIsolatedTerminal } from './test-helpers';
 
 describe('Buffer API', () => {
   let term: Terminal | null = null;
   let container: HTMLElement | null = null;
-  let ghostty: Ghostty | null = null;
 
   beforeEach(async () => {
-    // Create a fresh Ghostty WASM instance for complete test isolation.
-    // This ensures no shared state between tests and allows parallel execution.
-    ghostty = await Ghostty.load();
-
     // Create a container element if document is available
     if (typeof document !== 'undefined') {
       container = document.createElement('div');
       document.body.appendChild(container);
-      term = new Terminal({ cols: 80, rows: 24, ghostty });
+      term = await createIsolatedTerminal({ cols: 80, rows: 24 });
       term.open(container);
     }
   });

lib/input-handler.test.ts

@@ -2,7 +2,7 @@
  * Unit tests for InputHandler
  */
 
-import { beforeAll, beforeEach, describe, expect, mock, test } from 'bun:test';
+import { beforeEach, describe, expect, mock, test } from 'bun:test';
 import { Ghostty } from './ghostty';
 import { InputHandler } from './input-handler';
 import { Key, KeyAction, Mods } from './types';
@@ -127,13 +127,10 @@ describe('InputHandler', () => {
   let dataReceived: string[];
   let bellCalled: boolean;
 
-  beforeAll(async () => {
-    // Load WASM once for all tests (expensive operation)
-    // wasmPath is now optional - auto-detected
+  beforeEach(async () => {
+    // Create a fresh Ghostty WASM instance for complete test isolation
     ghostty = await Ghostty.load();
-  });
 
-  beforeEach(() => {
     // Create mock container for each test
     container = createMockContainer();