feat(lifecycle): plain-language generation diffs (#82)

nomarchy-what-changed wraps nvd into "N added, M removed, K updated"
summaries. nomarchy-rebuild / nomarchy-home print full nvd and toast
the one-liner; System › What changed? offers toast + floating report.
checks.what-changed fixtures nvd via NOMARCHY_NVD.

Verified: V2 — flake check --no-build; checks.what-changed green;
local smoke on real HM generations.

pkgs/nomarchy-lifecycle/default.nix

@@ -2,7 +2,7 @@
 # Installed on both NixOS (systemPackages) and Home Manager so a home
 # switch can refresh them without waiting for a full system rebuild —
 # otherwise a broken nomarchy-pull can never update itself.
-{ lib, writeShellScriptBin, nvd, jq, symlinkJoin }:
+{ lib, writeShellScriptBin, nvd, jq, symlinkJoin, nomarchy-what-changed }:
 
 let
   # Shared preamble: refuse root, resolve flake path.
@@ -78,6 +78,13 @@ let
     else
       echo "nomarchy-rebuild: what changed:"
       ${nvd}/bin/nvd diff "$before" "$after" || true
+      # One-line toast for the session (full report already on the terminal).
+      if command -v notify-send >/dev/null 2>&1 \
+         && command -v nomarchy-what-changed >/dev/null 2>&1; then
+        body=$(nomarchy-what-changed --summary --diff "$before" "$after" 2>/dev/null \
+               | sed 's/^Diff: //' || true)
+        [ -n "''${body:-}" ] && notify-send -a Nomarchy "System rebuild" "$body" || true
+      fi
     fi
   '';
 
@@ -87,6 +94,16 @@ let
       echo "nomarchy-home: no flake.nix at $flake" >&2
       exit 1
     fi
+    # Snapshot the active HM generation before the switch so we can nvd it.
+    hm_before=""
+    for d in \
+      "''${XDG_STATE_HOME:-$HOME/.local/state}/nix/profiles" \
+      "/nix/var/nix/profiles/per-user/$(id -un)"; do
+      if [ -L "$d/home-manager" ]; then
+        hm_before=$(readlink -f "$d/home-manager" 2>/dev/null || true)
+        break
+      fi
+    done
     log=$(mktemp)
     trap 'rm -f "$log"' EXIT
     set +e
@@ -102,7 +119,27 @@ let
       echo "Recovery:  home-manager generations  (or docs/RECOVERY.md)"
       exit "$rc"
     fi
-    echo "nomarchy-home: desktop applied."
+    hm_after=""
+    for d in \
+      "''${XDG_STATE_HOME:-$HOME/.local/state}/nix/profiles" \
+      "/nix/var/nix/profiles/per-user/$(id -un)"; do
+      if [ -L "$d/home-manager" ]; then
+        hm_after=$(readlink -f "$d/home-manager" 2>/dev/null || true)
+        break
+      fi
+    done
+    if [ -n "$hm_before" ] && [ -n "$hm_after" ] && [ "$hm_before" != "$hm_after" ]; then
+      echo "nomarchy-home: what changed:"
+      ${nvd}/bin/nvd diff "$hm_before" "$hm_after" || true
+      if command -v notify-send >/dev/null 2>&1 \
+         && command -v nomarchy-what-changed >/dev/null 2>&1; then
+        body=$(nomarchy-what-changed --summary --diff "$hm_before" "$hm_after" 2>/dev/null \
+               | sed 's/^Diff: //' || true)
+        [ -n "''${body:-}" ] && notify-send -a Nomarchy "Desktop updated" "$body" || true
+      fi
+    else
+      echo "nomarchy-home: desktop applied."
+    fi
   '';
 
   # Legacy aliases
@@ -125,6 +162,7 @@ symlinkJoin {
     sys-update
     sys-rebuild
     home-update
+    nomarchy-what-changed
   ];
   meta = {
     description = "Nomarchy machine-flake lifecycle: pull, rebuild, home";