docs(roadmap): note btrfs-assistant segfault; snapper backend VM-verified

A VM regression test caught that btrfs-assistant 2.2 segfaults on launch
inside libbtrfsutil.so.1.4.0 -- a library ABI mismatch in nixpkgs 26.05, not
a GL/VM artifact (crashes with software GL and every Qt platform; the fault
is in a btrfs library). The snapshot menu wiring is correct and the app is
installed, but the GUI doesn't open. The snapshot backend is unaffected and
is itself VM-verified (full BTRFS+LUKS install: root+home snapper configs, a
/home timeline snapshot, /home/.snapshots a real subvolume, the oneshot
Result=success). Keep the entry, document the bug pending a nixpkgs fix.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -327,8 +327,20 @@ how to override it. Items marked ✓ are shipped.
   snapper to snapshot **/home** by default (hourly 5 / daily 7 / weekly 4)
   when it's its own BTRFS subvolume; a boot oneshot creates the required
   `/home/.snapshots` subvolume if missing (covers existing installs, no disko
-  change). Remaining (optional): a keyboard-driven rofi browse for quick
-  glances; boot-from-snapshot needs a systemd-boot equivalent of grub-btrfs.
+  change). **The backend is VM-verified** (a full BTRFS+LUKS install: both
+  `root`+`home` configs, a `/home` timeline snapshot taken, `/home/.snapshots`
+  a real subvolume, the oneshot `Result=success`).
+  - ⚠ **Known bug — btrfs-assistant 2.2 segfaults on launch** in nixpkgs
+    26.05: it crashes inside `libbtrfsutil.so.1.4.0` (a library ABI mismatch,
+    confirmed in the VM regardless of GL/Qt platform — so it's not a VM/GL
+    artifact and will crash on hardware too). The menu wiring is correct and
+    the app is installed, but the GUI doesn't open. The snapshot *backend*
+    (the actual snapshots) is unaffected. Fix path: a nixpkgs override aligning
+    its libbtrfsutil, or wait for an upstream bump; meanwhile the menu entry
+    is a no-op when the binary crashes. Fallback if it lingers: a rofi-based
+    snapshot menu (no btrfs-assistant dependency).
+  - Remaining (optional): a keyboard-driven rofi browse for quick glances;
+    boot-from-snapshot needs a systemd-boot equivalent of grub-btrfs.
 - **Update awareness:** updates are manual today (`sys-update`/`home-update`);
   add a Waybar indicator / notification when flake inputs are stale or a new
   nixpkgs rev is available (optionally with a pending-change count). Augments,