Use low alignment bits instead of truncating high pointer bits

Changed PackedNext implementation to preserve ARM TBI/PAC/MTE pointer metadata:
- Store full usize pointer with auto_delete flag in LOW alignment bit (not high bit)
- ConcurrentTask is 8-byte aligned (contains u64 Task), so low bit is guaranteed zero
- Use enum(usize) { zero = 0, _ } for type safety and clean initialization syntax
- All atomic operations use usize (instead of u64) with @intFromEnum/@enumFromInt
- Masks: AUTO_DELETE_MASK=0x1, POINTER_MASK=~0x1

Benefits:
- Preserves all high pointer metadata (ARM pointer authentication, tagging, etc.)
- Uses guaranteed-zero low alignment bits instead of truncating address space
- Type-safe enum wrapper prevents accidental misuse
- Clean .zero initialization syntax

src/bun.js/event_loop/JSCScheduler.zig

@@ -27,7 +27,7 @@ export fn Bun__queueJSCDeferredWorkTaskConcurrently(jsc_vm: *VirtualMachine, tas
     var loop = jsc_vm.eventLoop();
     const concurrent_task = ConcurrentTask.new(.{
         .task = Task.init(task),
-        .next = .{},
+        .next = .zero,
     });
     concurrent_task.next.setAutoDelete(true);
     loop.enqueueTaskConcurrent(concurrent_task);