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>

agent/MEMORY.md

@@ -31,10 +31,14 @@ iteration would otherwise rediscover.
 - No KVM = slow, not broken; don't read slowness as failure.
 
 ## Known-broken / watchlist
-- **btrfs-assistant 2.2 segfaults on launch** (nixpkgs 26.05,
-  `libbtrfsutil.so.1.4.0` ABI mismatch; crashes on hardware too, not a VM
-  artifact). `nomarchy-snapshots` fzf flow is the shipped fallback;
-  re-check on every lock bump (§ Snapshot browse/restore; BACKLOG NOW#2).
+- **btrfs-assistant "segfault" was unprivileged-only** (re-diagnosed
+  2026-07-04): libbtrfsutil's unprivileged subvolume iteration crashes on
+  btrfs-progs 6.17.1 (upstream-fixed after); **as root it works**, and the
+  pkexec launcher runs it as root. The real distro bug was **no polkit
+  agent in the session** (every pkexec failed silently) — hyprpolkitagent
+  now ships (hyprland.nix exec-once). `checks.snapshot-gui` guards the
+  root path. Lesson: before "app X is broken", check WHO it runs as — and
+  whether polkit prompts can render at all (§ Snapshot browse/restore).
 - **NixOS release bump is a trap:** the discarded attempt
   (branch deleted 2026-06-22) hit a Hyprland OOM blocker; a redo is a
   deliberate `v2`, never part of routine lock bumps.