feat(idols-ai): enable zswap with zstd and 10% pool

Compressed writeback cache in front of the disk swapfile keeps cold anon
pages in RAM instead of the SSD, freeing page cache for the ~78GB mmap'd
LLM weights. zstd's ratio keeps ~5G more anon bytes in the capped pool
than lz4 at negligible cost (CPU peaks ~30% on this host); pool capped at
10% so it never competes with the weight cache.

hosts/idols-ai/default.nix

@@ -42,6 +42,20 @@ in
   # Disable the whole module (zram device + its swappiness=180 sysctl tunings); this host uses a disk swapfile instead.
   modules.zram.enable = false;
 
+  # zswap: compressed writeback cache in front of the disk swapfile.
+  # Keeps swapped cold anon pages compressed in RAM instead of the SSD, which frees more
+  # page cache for the ~78GB mmap'd LLM weights (mmap file pages never go through zswap).
+  # Safe here: disk swapfile remains the real backing store, no zram deadlock structure.
+  # - zstd: CPU is not the bottleneck here (llama.cpp peaks ~30% util); its ~2.5x ratio
+  #   keeps ~5G more anon bytes in the capped pool than lz4, and ~5us decompress is noise
+  #   vs the ~100us SSD fault it avoids.
+  # - 10% pool (~9G): the module default 25% (~23G) would compete with the weight cache.
+  boot.zswap = {
+    enable = true;
+    compressor = "zstd";
+    maxPoolPercent = 10;
+  };
+
   services.sunshine.enable = false;
   services.tuned.ppdSettings.main.default = lib.mkForce "performance";