Move cache into Buffer owned object, use WeakRefs

This allows O(1) clearing of the cache, a single TTL timer and we
can avoid making BufferLine's Disposable

src/common/buffer/BufferLineStringCache.ts

@@ -0,0 +1,37 @@
+/**
+ * Copyright (c) 2026 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import type { IBufferLineStringCache, IBufferLineStringCacheEntry } from 'common/buffer/BufferLine';
+
+export class BufferLineStringCache implements IBufferLineStringCache {
+  public generation: number = 0;
+  public readonly entries: Set<IBufferLineStringCacheEntry> = new Set();
+
+  constructor(private readonly _onCacheUsed: () => void = () => { }) { }
+
+  public touch(): void {
+    this._onCacheUsed();
+  }
+
+  public allocateEntry(): IBufferLineStringCacheEntry {
+    const entry: IBufferLineStringCacheEntry = {
+      value: undefined,
+      isTrimmed: false,
+      generation: this.generation
+    };
+    this.entries.add(entry);
+    this._onCacheUsed();
+    return entry;
+  }
+
+  public clear(): void {
+    this.generation++;
+    for (const entry of this.entries) {
+      entry.value = undefined;
+      entry.isTrimmed = false;
+    }
+    this.entries.clear();
+  }
+}