say the store check is an allowlist: only ext4, btrfs and xfs pass

docs/agent-lifecycle/persistence.md

@@ -489,10 +489,14 @@ hive-c0re can't read, or none, leaves the agent as it is.
   `claude/` dirs and its socket dir with `owneridmap`, and every other bind too
   (config, `/knowledge`, the runtime dir), whose owner shows as root inside.
 - **`/nix/store` has to be on ext4, btrfs or xfs.** nspawn mounts the store
-  into a namespaced container idmapped, which other filesystems (overlayfs,
+  into a namespaced container idmapped, and the kernel refuses that on
-  tmpfs, older zfs) refuse. `WriteNspawnFlags` for an opted-in agent checks
+  overlayfs and on some other filesystems. `WriteNspawnFlags` for an opted-in
-  the `statfs` type of `/nix/store` and fails with the type and the fix,
+  agent checks the `statfs` type of `/nix/store` against an allowlist of those
-  before it moves anything. Check a host with `findmnt -no FSTYPE /nix/store`.
+  three. The check refuses every other filesystem, zfs and tmpfs included, even
+  where a newer kernel could idmap it, because the gate exists to stop a
+  container that would not boot. The error names the type and the fix, and
+  comes before anything moves. Check a host with
+  `findmnt -no FSTYPE /nix/store`.
 - **The container starts with `--private-users=pick --private-users-ownership=map`.**
   nspawn idmaps the root at mount time, so a different range for `pick` never
   walks or chowns it.

hive-priv/src/userns.rs

@@ -51,7 +51,7 @@ const STORE: &str = "/nix/store";
 type FsType = libc::__fsword_t;
 
 /// `f_type` of the filesystems that support idmapped mounts and that a store
-/// can sit on: ext4, btrfs and xfs.
+/// can sit on: ext4, btrfs and xfs. Nothing else is accepted.
 const IDMAP_FS: [FsType; 3] = [0xEF53, 0x9123_683E, 0x5846_5342];
 
 /// The dirs under `<AGENT_STATE_ROOT>/<agent>` the agent writes into. `config`
@@ -111,18 +111,19 @@ fn fs_type(path: &Path) -> std::io::Result<FsType> {
     Ok(buf.f_type)
 }
 
-/// Refuse a store on a filesystem that cannot be idmapped: nspawn clones a
+/// Refuse a store whose filesystem is not ext4, btrfs or xfs. nspawn clones a
 /// namespaced container's `/nix/store` bind with an idmap, which the kernel
-/// refuses there, so the container would not boot.
+/// refuses on other filesystems, so the container would not boot. Others are
+/// refused even where a newer kernel could idmap them.
 fn check_store(agent: &str, f_type: std::io::Result<FsType>) -> Result<()> {
     let f_type = f_type.with_context(|| format!("{agent}: statfs {STORE}"))?;
     if IDMAP_FS.contains(&f_type) {
         return Ok(());
     }
     bail!(
-        "{agent} sets privateUsers, but {STORE} is on a filesystem (f_type {f_type:#x}) that \
+        "{agent} sets privateUsers, but {STORE} is on a filesystem (f_type {f_type:#x}) other \
-         cannot be idmapped; move {STORE} to ext4, btrfs or xfs, or leave privateUsers off for \
+         than ext4, btrfs or xfs, the only ones accepted; move {STORE} to one of them, or leave \
-         this agent"
+         privateUsers off for this agent"
     )
 }