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/bake/DevServer/WatcherAtomics.zig

@@ -132,7 +132,7 @@ pub fn watcherReleaseAndSubmitEvent(self: *Self, ev: *HotReloadEvent) void {
                 self.dbg_server_event = ev;
             }
             ev.concurrent_task = .{
-                .next = .{},
+                .next = .zero,
                 .task = jsc.Task.init(ev),
             };
             ev.concurrent_task.next.setAutoDelete(false);