fix concurrent-modification in BufferLine anchor iteration

`removeCells`, `insertCells`, and `dispose` each iterate over
`_anchors` while invoking `anchor.dispose()` on entries inside the
loop — but `dispose()` removes the anchor from the same list, which
shifts later indexes left and causes the for-loop to skip them.

Symptoms (no user-facing report yet, but real correctness bug):
- After `removeCells` with multiple anchors past the start, anchors
  that should be repositioned were silently left at their old `x`.
- After `insertCells` with anchors getting pushed past `_length`,
  ones meant to be disposed could survive.
- `BufferLine.dispose` would throw `ConcurrentModificationError` as
  soon as more than one anchor was attached.

Fix: iterate `_anchors.toList()` (a snapshot) in all three sites.
Cheap, safe, and matches the expected anchor-management semantics.

Surfaced by the unit tests added under T-91; that commit covers the
fix with regression tests.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/src/terminal/src/core/buffer/line.dart

@@ -179,8 +179,9 @@ class BufferLine with IndexedItem {
     }
 
     // Update anchors, remove anchors that are inside the removed range.
-    for (var i = 0; i < _anchors.length; i++) {
-      final anchor = _anchors[i];
+    // Iterate over a snapshot — anchor.dispose() removes itself from
+    // _anchors, so a live for-loop would skip later entries.
+    for (final anchor in _anchors.toList()) {
       if (anchor.x >= start) {
         if (anchor.x < start + count) {
           anchor.dispose();
@@ -218,8 +219,9 @@ class BufferLine with IndexedItem {
     }
 
     // Update anchors, move anchors that are after the inserted range.
-    for (var i = 0; i < _anchors.length; i++) {
-      final anchor = _anchors[i];
+    // Iterate over a snapshot — anchor.dispose() removes itself from
+    // _anchors, so a live for-loop would skip later entries.
+    for (final anchor in _anchors.toList()) {
       if (anchor.x >= start + count) {
         anchor.reposition(anchor.x + count);
 
@@ -352,7 +354,8 @@ class BufferLine with IndexedItem {
   }
 
   void dispose() {
-    for (final anchor in _anchors) {
+    // Snapshot — anchor.dispose() removes itself from _anchors.
+    for (final anchor in _anchors.toList()) {
       anchor.dispose();
     }
   }