fix: disk picker fd0 (#112), menu Back guard (#106), offline theme (#113)

- #112: list_installable_disks drops fd/loop/sr/zram/<8GiB, largest-first
  so OVMF no longer offers /dev/fd0 as the default wipe target
- #106: tools/check-menu-back.py + checks.menu-back (proved to fail)
- #113: offline theme-switch contract documented (default/pinned only);
  theme-sync run_switch adds offline-oriented hint when no network

Verified: V0 flake check; installer-safety; menu-back; py_compile.

agent/MEMORY.md

@@ -137,6 +137,11 @@ iteration would otherwise rediscover.
   without a hierarchy pass.
 
 ## Gotchas (cost a debugging session once)
+- **OVMF exposes /dev/fd0 as TYPE=disk (#112):** disk picker must exclude
+  `/dev/fd*` and tiny sizes; sort largest-first or blind Enter picks floppy.
+- **Live ISO offline theme switch (#113):** only the *default/pinned* HM
+  generation is offline-safe; other presets may try to build the world —
+  document the contract and fail with a network-oriented message.
 - **Installer HM pre-activate needs XDG_RUNTIME_DIR (+ session bus) (#123):**
   `runuser … activate` inside `nixos-enter` has no user session. Without
   `mkdir -p /run/user/$UID` owned by the install user and