webapi: tighten DOM allocation lifetimes

Use the call arena only for the AbortSignal dependent list that must survive re-entrant abort handlers, and move scratch-only DOM reads to the local arena. Store BeforeUnloadEvent.returnValue in the event-owned arena so it remains valid without accumulating in the longer-lived frame arena.

These allocator changes bound temporary memory to the shortest safe lifetime while preserving existing Web API behavior. Add coverage for replacing BeforeUnloadEvent.returnValue.

src/browser/tests/events.html

@@ -940,3 +940,16 @@
     child.removeEventListener('click', listener);
   }
 </script>
+
+<script id=beforeUnloadEventReturnValue>
+  {
+    const event = document.createEvent('BeforeUnloadEvent');
+    testing.expectEqual('', event.returnValue);
+
+    event.returnValue = 'first';
+    testing.expectEqual('first', event.returnValue);
+
+    event.returnValue = 'replacement';
+    testing.expectEqual('replacement', event.returnValue);
+  }
+</script>

src/browser/webapi/AbortSignal.zig

@@ -106,7 +106,7 @@ pub fn abort(self: *AbortSignal, reason_: ?Reason, exec: *const Execution) !void
     var to_dispatch: std.ArrayList(Dependend) = .{};
     for (self._dependents.items) |dep| {
         if (try dep.markAborted(self._reason, exec)) {
-            try to_dispatch.append(exec.arena, dep);
+            try to_dispatch.append(exec.call_arena, dep);
         }
     }
 

src/browser/webapi/element/Html.zig

@@ -380,7 +380,7 @@ pub fn getAccessKeyLabel(self: *HtmlElement, frame: *Frame) ![]const u8 {
     if (codepoints != 1) {
         return "";
     }
-    return std.fmt.allocPrint(frame.call_arena, "Alt+{s}", .{value});
+    return std.fmt.allocPrint(frame.local_arena, "Alt+{s}", .{value});
 }
 
 pub fn getPopover(self: *HtmlElement) ?[]const u8 {

src/browser/webapi/event/BeforeUnloadEvent.zig

@@ -71,8 +71,8 @@ pub fn getReturnValue(self: *const BeforeUnloadEvent) []const u8 {
     return self._return_value;
 }
 
-pub fn setReturnValue(self: *BeforeUnloadEvent, value: []const u8, frame: *Frame) !void {
-    self._return_value = try frame.dupeString(value);
+pub fn setReturnValue(self: *BeforeUnloadEvent, value: []const u8) !void {
+    self._return_value = try self._proto._arena.dupe(u8, value);
 }
 
 pub const JsApi = struct {