fix(nixos): gate zram kernel sysctl tunings behind modules.zram.enable

The zram module was auto-imported on all hosts via base scanPaths, and its
swappiness=180/page-cluster=0 tunings leaked onto hosts that disabled zram
(e.g. idols-ai with a disk swapfile, pushing cold anon pages to disk swap).
Wrap the whole module in modules.zram.enable (default true); idols-ai now
turns off zram and its tunings with a single switch.

hosts/idols-ai/default.nix

@@ -39,7 +39,8 @@ in
   ];
 
   # Zram consumes physical memory for compression, which can cause a deadlock and system hang if the model size approaches the physical memory limit.
-  zramSwap.enable = lib.mkForce false;
+  # Disable the whole module (zram device + its swappiness=180 sysctl tunings); this host uses a disk swapfile instead.
+  modules.zram.enable = false;
 
   services.sunshine.enable = false;
   services.tuned.ppdSettings.main.default = lib.mkForce "performance";

modules/nixos/base/zram.nix

@@ -1,5 +1,28 @@
-{ lib, ... }:
 {
+  lib,
+  config,
+  ...
+}:
+let
+  cfg = config.modules.zram;
+in
+{
+  options.modules.zram.enable =
+    lib.mkEnableOption ''
+      zram in-memory swap device plus kernel sysctl tunings for it.
+
+      The sysctl tunings (swappiness=180 etc.) are designed for in-memory swap
+      devices only; they are gated behind this switch so that hosts which
+      disable zram (e.g. using a disk swapfile instead) also revert to the
+      kernel default swappiness instead of leaking zram tunings onto disk swap.
+
+      Enabled by default; hosts that want disk-only swap should set it to false.
+    ''
+    // {
+      default = true;
+    };
+
+  config = lib.mkIf cfg.enable {
     # Enable in-memory compressed devices and swap space provided by the zram kernel module.
     # By enable this, we can store more data in memory instead of fallback to disk-based swap devices directly,
     # and thus improve I/O performance when we have a lot of memory.
@@ -15,7 +38,7 @@
       priority = lib.mkDefault 100;
       # Maximum total amount of memory that can be stored in the zram swap devices (as a percentage of your total memory).
       # Defaults to 1/2 of your total RAM. Run zramctl to check how good memory is compressed.
-    # This doesn’t define how much memory will be used by the zram swap devices.
+      # This doesn't define how much memory will be used by the zram swap devices.
       memoryPercent = lib.mkDefault 50;
     };
 
@@ -27,7 +50,7 @@
 
       # vm.watermark_boost_factor - Controls aggressiveness of memory reclaim (default: 15000)
       # Setting to 0 disables watermark boost, preventing premature memory reclamation.
-    # This allows fuller memory utilization before the kernel starts reclaiming pages.
+      # This allows fuller memory utilization before the kernel starts reclaiming.
       "vm.watermark_boost_factor" = lib.mkDefault 0;
 
       # vm.watermark_scale_factor - Controls kswapd wakeup frequency (range: 1-1000, default: 10)
@@ -41,4 +64,5 @@
       # For low-latency devices like zram, readahead hurts performance by fetching unnecessary data.
       "vm.page-cluster" = lib.mkDefault 0;
     };
+  };
 }