fix(snapshots): un-break the GUI — polkit agent + launcher, not an override

Re-diagnosis of the "btrfs-assistant 2.2 segfaults on launch" bug, and
the fix is not the planned nixpkgs override:

1. gdb: the crash is btrfs_util_subvolume_iterator_next() —
   libbtrfsutil's UNPRIVILEGED subvolume-iteration path in btrfs-progs
   6.17.1 (upstream-fixed after 6.17.1, kdave/btrfs-progs 886571653).
   Symbol versions were checked and match: not ABI/link drift.
2. VM A/B on real btrfs: as root it works (exit 0); as a user it
   segfaults (139). The pkexec launcher runs it as root — so the GUI
   path was fine all along, except…
3. …the distro shipped NO polkit authentication agent: every pkexec
   prompt in the session failed silently. That is the actual root
   cause of "the GUI doesn't open".

Shipped: hyprpolkitagent (Hyprland's Qt agent — the prompt is
Stylix-themed) via exec-once in hyprland.nix; `nomarchy-menu snapshot`
prefers btrfs-assistant-launcher again, with the nomarchy-snapshots
fzf flow kept as fallback (and for SSH); checks.snapshot-gui guards
the root path + offscreen GUI event loop on a real btrfs volume so a
lock bump can't silently regress it. No btrfs-progs patch: all our
flows are root-side; the unprivileged fix arrives with a lock bump.

Verified: V0 (flake check) + V2 (snapshot-gui check, executed green) +
V1 (HM generation builds; hyprland.conf carries the agent exec-once;
generated menu passes bash -n with the launcher wiring). Remaining V3
queued: first on-hardware themed polkit prompt.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/hyprland.nix

@@ -200,6 +200,12 @@ in
         # Paint the wallpaper as soon as the session is up (waits for
         # the daemon internally).
         "nomarchy-theme-sync wallpaper"
+        # Polkit authentication agent — without one, EVERY pkexec/polkit
+        # prompt in the session fails silently (btrfs-assistant-launcher
+        # was the discovery case). hyprpolkitagent is Hyprland's own Qt
+        # agent, so the prompt is Stylix-themed like every other Qt
+        # surface. Its binary lives in libexec (not bin), hence the path.
+        "${pkgs.hyprpolkitagent}/libexec/hyprpolkitagent"
         # Waybar is launched here rather than as a systemd user service: bound
         # to graphical-session.target the unit raced Hyprland's IPC on relogin
         # and landed in `failed`; exec-once only fires once the compositor is

modules/home/rofi.nix

@@ -404,11 +404,17 @@ ${themeRows}
         exit 0 ;;
 
       snapshot)
-        # btrfs-assistant 2.2 segfaults on 26.05 (libbtrfsutil ABI), so launch
-        # the keyboard-driven snapper browser instead: a terminal running it
-        # via sudo (snapper is root-only here — one password prompt), with
-        # browse/diff, restore (undochange) and rollback, each confirmed.
+        # Snapshot browse/restore — the btrfs-assistant GUI via its pkexec
+        # launcher. The "2.2 segfaults on 26.05" diagnosis was HALF right:
+        # only UNPRIVILEGED runs crash (libbtrfsutil unprivileged subvolume
+        # iteration, btrfs-progs 6.17.1; fixed upstream after) — as root it
+        # runs fine, and the launcher runs it as root. The prompt needs the
+        # session polkit agent (hyprpolkitagent, exec-once in hyprland.nix).
+        # Fallback: nomarchy-snapshots, the keyboard-driven fzf browser in a
+        # terminal (kept installed — also nice over SSH).
         # Self-gated to nomarchy.system.snapper, so it no-ops elsewhere.
+        command -v btrfs-assistant-launcher >/dev/null 2>&1 \
+          && exec btrfs-assistant-launcher
         command -v nomarchy-snapshots >/dev/null 2>&1 \
           || { notify-send "Snapshots" "Snapshot tools unavailable (nomarchy.system.snapper off?)."; exit 0; }
         exec ${cfg.terminal} -e sudo nomarchy-snapshots ;;

modules/nixos/default.nix

@@ -344,13 +344,17 @@ in
         nixos-rebuild switch --flake /etc/nixos#default "$@"
       '')
     # The desktop snapshot manager (browse / diff / restore / rollback over
-    # snapper, elevating via polkit). Kept for when nixpkgs fixes its crash.
+    # snapper, elevating via polkit) — the primary `nomarchy-menu snapshot`
+    # target. The "2.2 segfault" is unprivileged-only (libbtrfsutil
+    # unprivileged subvolume iteration, btrfs-progs 6.17.1, fixed upstream
+    # after); the pkexec launcher runs it as root, where it works —
+    # VM-proven, guarded by checks.snapshot-gui.
     ++ lib.optional cfg.snapper.enable pkgs.btrfs-assistant
-    # Keyboard-driven snapper browser/restore — the fallback `nomarchy-menu
-    # snapshot` actually launches while btrfs-assistant 2.2 segfaults on 26.05
-    # (libbtrfsutil ABI). Runs as root (snapper is root-only here; the menu
-    # opens it in a terminal via sudo, one password prompt), fzf to pick, with
-    # browse/diff (read-only) and typed-`yes` confirmation before any write.
+    # Keyboard-driven snapper browser/restore — the menu's fallback when the
+    # GUI is absent, and handy over SSH. Runs as root (snapper is root-only
+    # here; the menu opens it in a terminal via sudo, one password prompt),
+    # fzf to pick, with browse/diff (read-only) and typed-`yes` confirmation
+    # before any write.
     ++ lib.optional cfg.snapper.enable (pkgs.writeShellApplication {
       name = "nomarchy-snapshots";
       runtimeInputs = with pkgs; [ snapper fzf gawk gnugrep less coreutils systemd ];