shell: add gpumean, a settled GPU busy sampler

settings/shell.nix

@@ -153,5 +153,40 @@ in
     end
 
     function fish_right_prompt; end
+
+    # GPU load, averaged. A single read of gpu_busy_percent is worthless —
+    # it catches whatever transient the card was in at that instant. Settle
+    # first, then sample, and report the spread as well as the mean: a
+    # steady 30% and a 0-to-70% sawtooth are different problems.
+    function gpumean --description "Mean GPU busy % — 5s settle, then N seconds at 5Hz (default 10)"
+      set -l secs 10
+      test (count $argv) -gt 0; and set secs $argv[1]
+      # find, not a glob: an unmatched wildcard is an error in fish and would
+      # abort the function before the emptiness check below.
+      set -l cards (find /sys/class/drm -maxdepth 3 -name gpu_busy_percent 2>/dev/null | sort)
+      if test (count $cards) -eq 0
+        echo "no gpu_busy_percent found (amdgpu/i915 only — nvidia doesn't expose it)"
+        return 1
+      end
+      sleep 5
+      set -l sum; set -l lo; set -l hi
+      for c in $cards
+        set -a sum 0; set -a lo 100; set -a hi 0
+      end
+      set -l n (math "$secs * 5")
+      for i in (seq $n)
+        for j in (seq (count $cards))
+          set -l v (cat $cards[$j])
+          set sum[$j] (math "$sum[$j] + $v")
+          test $v -lt $lo[$j]; and set lo[$j] $v
+          test $v -gt $hi[$j]; and set hi[$j] $v
+        end
+        sleep 0.2
+      end
+      for j in (seq (count $cards))
+        set -l name (string replace -r '.*/(card[0-9]+)/.*' '$1' $cards[$j])
+        echo "$name mean="(math -s0 "$sum[$j] / $n")"% min=$lo[$j]% max=$hi[$j]% ($secs""s)"
+      end
+    end
   '';
 }