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/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 ];