Split common into primitives and runtime layers

Reorganize src/common per issue #5963: move building blocks under
primitives/ and services/integration under runtime/. Preserve existing
common/* import paths via TypeScript path mapping. Add src/ARCHITECTURE.md
documenting modules and goals.

Fix two relative imports broken by the directory depth change.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/common/primitives/Event.test.ts

@@ -0,0 +1,72 @@
+/**
+ * Copyright (c) 2024-2026 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { assert } from 'chai';
+import { Emitter } from 'common/Event';
+
+describe('Emitter', () => {
+  it('should fire with 0 listeners without error', () => {
+    const emitter = new Emitter<number>();
+    emitter.fire(42);
+  });
+
+  it('should fire with 1 listener', () => {
+    const emitter = new Emitter<number>();
+    let received: number | undefined;
+    emitter.event(e => { received = e; });
+    emitter.fire(42);
+    assert.strictEqual(received, 42);
+  });
+
+  it('should fire with 1 listener using thisArgs', () => {
+    const emitter = new Emitter<number>();
+    const obj = { value: 0, handler(e: number) { this.value = e; } };
+    emitter.event(obj.handler, obj);
+    emitter.fire(42);
+    assert.strictEqual(obj.value, 42);
+  });
+
+  it('should fire with multiple listeners', () => {
+    const emitter = new Emitter<number>();
+    const results: number[] = [];
+    emitter.event(e => results.push(e * 1));
+    emitter.event(e => results.push(e * 2));
+    emitter.event(e => results.push(e * 3));
+    emitter.fire(10);
+    assert.deepEqual(results, [10, 20, 30]);
+  });
+
+  it('should handle listener removal during fire', () => {
+    const emitter = new Emitter<number>();
+    const results: string[] = [];
+    emitter.event(() => results.push('first'));
+    const disposable = emitter.event(() => {
+      results.push('second');
+      disposable.dispose();
+    });
+    emitter.event(() => results.push('third'));
+    emitter.fire(1);
+    assert.deepEqual(results, ['first', 'second', 'third']);
+  });
+
+  it('should not fire after dispose', () => {
+    const emitter = new Emitter<number>();
+    let called = false;
+    emitter.event(() => { called = true; });
+    emitter.dispose();
+    emitter.fire(42);
+    assert.strictEqual(called, false);
+  });
+
+  it('should allow disposing a listener', () => {
+    const emitter = new Emitter<number>();
+    let count = 0;
+    const disposable = emitter.event(() => { count++; });
+    emitter.fire(1);
+    disposable.dispose();
+    emitter.fire(2);
+    assert.strictEqual(count, 1);
+  });
+});