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/JOURNAL.md

@@ -17,6 +17,24 @@ Template:
 
 ---
 
+## 2026-07-04 — Snapshot GUI un-broken + polkit agent (loop iteration #4)
+- **Task:** BACKLOG NOW#4 — btrfs-assistant segfault.
+- **Did:** re-diagnosed instead of overriding. gdb: crash is in
+  libbtrfsutil's *unprivileged* subvolume iteration (btrfs-progs 6.17.1;
+  upstream fix `886571653` post-6.17.1; symbol versions verified — not
+  ABI drift). VM A/B: root works (exit 0), user crashes (139). The
+  launcher runs it as root → GUI fine *if* pkexec can prompt — and the
+  distro shipped **no polkit agent** (all pkexec silently failed; the
+  actual root cause of "GUI is dead"). Shipped: hyprpolkitagent
+  (exec-once, Stylix-themed Qt), menu re-pointed to the launcher with
+  the fzf flow as fallback, `checks.snapshot-gui` canary (root path +
+  offscreen event loop on real btrfs). No btrfs-progs patch — root-side
+  flows don't hit the bug; the fix arrives with a lock bump.
+- **Verified:** V0 + V2 (snapshot-gui check, run for real) + V1 (HM
+  generation renders the agent exec-once; menu bash -n).
+- **Pending:** V3 queued — first on-hardware themed polkit prompt.
+- **Next suggestion:** NOW#5 (keyboard cycle bind + summer parity).
+
 ## 2026-07-04 — CI is green (run #58) — item 20 main half closed
 - **Task:** monitor follow-up, no code. Run #58 (the .gitea path fix)
   **succeeded** — first green CI: the container recipe works on the