fix(v4.0.0): storage -- what the review of step 6 found

A block request with fewer than two values on the stack is refused; it
had acted on whatever the stack ring held and stopped the node.

Bare metal: when the kernel's chain takes the place of POST's block RAM
the node's two buffers are emptied, so it no longer holds POST's copy of
a block; and the chain's fast RAM is cleared, so a node cannot read what
was in the kernel's heap.

blocks.c is built with each test under that test's own warnings and
sanitizers; it had been left out of both.  The hosted link cleans its
object directory first: it had linked the withdrawn store_v3.o left there
from the day before.

node.h and DECOMPOSITION.md D-19 no longer describe the message device or
the four registers as current.  MESH.md 8.5 records two findings for
ruling: a node's own copy of a block, and a block read over a node's code.

From a clean build: make -C v4 test, sanitize and hosted-check pass;
amd64, aarch64 and riscv64 boot, POST 538 of 538, same hashes, blocks 1
and 2047 clean at the prompt: logs/20261007-085017, -085254, -085636.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/system/blocks.c

@@ -15,6 +15,11 @@ int v4_blocks_serve(v4_node *n, v4_cell request)
     int status;
 
     if (request != V4_REQ_BLOCK_READ && request != V4_REQ_BLOCK_WRITE) return 0;
+    if (n->ds.depth < 2u) {                          /* not both arguments: nothing is done with what is there */
+        v4_dstack_reset(&n->ds);
+        v4_dstack_push(&n->ds, (v4_cell)V4_BLOCK_REFUSED);
+        return 1;
+    }
     addr = v4_dstack_pop(&n->ds);
     num = v4_dstack_pop(&n->ds);
     block = (uint32_t)num;