fix(p8-security): close review gaps in the ownership gate and copy-as failure propagation

- H3: a daemon module without 'client owner = yes' now also has super-user
  device activity forced off (char/block mknod, --write-devices), so a root
  daemon can no longer be made to create/write raw devices under AUTO.  The
  entries are skipped, preserving ordinary -a pushes.
- H1/H2: propagate a failed required --copy-as chown from symlink metadata
  restore and implicitly-created parent directories, so the entry (and run)
  reports failure instead of a wrong-owner success.
- Docs/help/headers updated for A2/A3 and the device clamp; startup warning
  spells out the client-owner risk.
- Tests: daemon device clamp (skipped without opt-in, created with opt-in),
  updated --super/--fake-super expectations.

src/shared/xattr.h

@@ -89,11 +89,13 @@ void fake_super_store_fd(int fd, uint32_t uid, uint32_t gid, uint32_t mode, int6
 /* --fake-super replay: parse the FAKESUPER_XATTR record previously written on
  * `fd` by fake_super_store_fd and re-apply uid/gid/mode/mtime fd-relative.
  * Best-effort: absence of the xattr or a malformed record is a silent no-op
- * that never fails the transfer; fchown is applied only when permitted (a
- * non-root EPERM/EACCES is skipped silently, matching FastSync's identity
- * philosophy), and the mode is sanitized exactly like the normal metadata path
- * (group/other write bits never granted).  Returns true when the xattr was
- * present and parsed. */
+ * that never fails the transfer.  The OWNER leg is applied only when an explicit
+ * ownership identity policy is active (numeric-ids/chown/usermap/groupmap/
+ * copy-as), when super-user activities are permitted, and when --copy-as is not
+ * authoritative; a non-root EPERM/EACCES is skipped silently, matching
+ * FastSync's identity philosophy.  The mode is sanitized exactly like the normal
+ * metadata path (group/other write bits never granted).  Returns true when the
+ * xattr was present and parsed. */
 bool fake_super_restore_fd(int fd);
 
 #endif