mount: use bcachefs -o X-mount.subdir={name} for subvolume mounts; update SAFETY notes; sync README and PROMPT with root/subvol scheme and bcachefs option

README.md

@@ -135,8 +135,18 @@ Defaults and policies
   btrfs (data) label: ZOSDATA
   bcachefs (data/cache) label: ZOSDATA
 - Mount scheme:
-  per-UUID under /var/cache/{UUID}
-  /etc/fstab generation is disabled by default
+  - Root mounts (runtime only): each data filesystem is mounted at /var/mounts/{UUID}
+    - btrfs root options: rw,noatime,subvolid=5
+    - bcachefs root options: rw,noatime
+  - Subvolume mounts (from the primary data filesystem only) to final targets:
+    - /var/cache/system
+    - /var/cache/etc
+    - /var/cache/modules
+    - /var/cache/vm-meta
+  - Subvolume mount options:
+    - btrfs: -o rw,noatime,subvol={name}
+    - bcachefs: -o rw,noatime,X-mount.subdir={name}
+  - /etc/fstab generation is disabled by default; when enabled, only the four subvolume mounts are written (UUID= sources, deterministic order)
 
 Tracing and logs
 - stderr logging level controlled by -l/--log-level (info by default)