store: refuse a device that does not carry the packed image's magic (inert, NOT installed)

This is the guard for one of the two doors on the walk-with-no-store defect:
"neither control block decodes" is not evidence of a packed image, so the packed
reader is allowed only for a device that begins with the image header's own magic —
the question the userspace raw reader has always asked and the kernel did not.

It builds clean and it does NOT stop the reproduction: `cube_store=/dev/null
cubelinux.enum=1` still serves invented records at the same rate. So the guard is
not on the path that answers, and it is committed for that reason stated plainly
rather than installed: this machine stays on #87, which is measured, instead of
moving to a kernel whose only change is one that has not been shown to do anything.

Kept rather than reverted because the check is right on its own terms — an
addressed store begins with CUBS, a bare packed image with CUBE — and because the
next attempt should begin by asking whether this guard is even reached before
writing another one.

drivers/cube/cubelinux_store.rs

@@ -220,6 +220,11 @@ const LOG_ALIGN: usize = 4096;
 /// Control block magic, distinct from the image's `CUBE` and the log's `CUBW`.
 const CTL_MAGIC: [u8; 4] = *b"CUBS";
 const CTL_VERSION: u8 = 1;
+/// The image header's own magic, which a bare packed image begins with.
+///
+/// It is read for one decision: a device whose control block does not decode is a packed image only
+/// if it says so. Without that question, anything at all was walked as though it were a store.
+const IMAGE_MAGIC: &[u8; 4] = b"CUBE";
 /// One control copy, and where the two live.
 const CTL_COPY_LEN: usize = 2048;
 const CTL_COPY_A: usize = 0;
@@ -3014,9 +3019,26 @@ impl Addressed {
             (Some(x), Some(y)) => core::cmp::max(x, y),
             (Some(x), None) => x,
             (None, Some(y)) => y,
-            // Neither copy decodes: there is no control block, so this is a bare image and the
-            // walking reader handles it.
-            (None, None) => return Ok(None),
+            // Neither copy decodes. That is *not* by itself evidence of a packed image: it is
+            // evidence that this device does not begin with a control block, and on a device that is
+            // not a store at all there is nothing to walk. The userspace raw reader has always asked
+            // that question — its message for this is `not a CUBE store (bad magic)` — and the kernel
+            // did not, which is the whole of this defect: with nothing behind the read, the walking
+            // reader served *invented* records, a garbage space with a zero-length value and a cursor
+            // that kept moving, for as long as the machine was left on. A boot pointed at a store it
+            // could not read hung instead of saying so.
+            //
+            // So the fallback is taken only for something that says it is a packed image: a bare
+            // packed image begins with the image header's magic. A device with anything else at
+            // offset 0 is refused here, loudly, which is what a missing or wrong store deserves.
+            (None, None) => {
+                let mut head = KVVec::<u8>::with_capacity(8, GFP_KERNEL)?;
+                read_exact_at(file, 0, IMAGE_MAGIC.len(), &mut head, &mut cache.scratch)?;
+                if head.as_slice() != IMAGE_MAGIC {
+                    return Err(EINVAL);
+                }
+                return Ok(None);
+            }
         } as u64;
 
         let mut opened = Addressed {