feat(v4.0.0): every node asks the kernel for its blocks; a born node is not POSTed

A block is a kernel request, as ENGINE.md 3.3 has it: the node puts the
block's number and the address of 256 cells on its stack and writes the
request to port 0, and the kernel leaves the status there.  The requests
are -1, read, and -2, write, the same for every node.  v4/system/blocks.c
serves them from the kernel's block subsystem, which is v3's.  The four
storage registers are gone from the engine.

The device that spoke block messages (4a505a15) is withdrawn with its
test and its message types: Captain Bob ruled on 2026-10-07 that it, a
node's own drive, and nodes with no storage had left the OS as designed
(docs/v4.0.0/MESH.md 8.5).

Hera no longer sends POST to the nodes she births: POST is the kernel's,
once.  Every node has its kernel on port 0; it serves a node's blocks and,
for Hera alone, her requests for nodes and capsules.

Bare metal: the node boots and is POSTed against POST's own block RAM,
and the kernel's chain -- fast RAM, the ramdrive, the virtio disk -- is
set up after POST and before the prompt, as on the v3 path.  The disk is
read and not written: nothing in v4 yet gives the owner's word that it
may be formatted.  A hosted program has the chain's fast RAM, as hosted
v3 has with no disk.  Error 17 is Storage refused.

make -C v4 test and sanitize pass at both widths; hosted-check passes on
three ISAs; amd64, aarch64 and riscv64 boot, POST 538 of 538, with the
typed session: logs/20261007-081603, -081839, -082226.  The hashes are
the same on all six.

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

kernel/src/v4/sk_v4.c

@@ -20,17 +20,21 @@
 #include "starkernel/console.h"
 #include "starkernel/arch.h"
 #include "v4/boot.h"
+#include "block_subsystem.h"
 
-#define SK_V4_BLOCKS 64u
 #define SK_V4_LINE   255u                /* what the line editor here takes */
 
 static v4_node       sk_v4_node;
 static v4_exec_state sk_v4_es;
 static v4_heat       sk_v4_heat;
-static unsigned char sk_v4_disk[SK_V4_BLOCKS * V4_BLOCK_BYTES];
 
 static char          sk_v4_text[SK_V4_LINE + 1u];
 
+/* POST's block RAM, as the v3 path has (sk_vm_bootstrap.c, sk_post_blk_ram):
+ * the node boots and is POSTed with these blocks, 1 to 2047 and nothing
+ * above them, and the kernel's real chain is set up afterwards. */
+static uint8_t       sk_v4_post_blk_ram[BLK_RAM_BLOCKS * BLK_FORTH_SIZE];
+
 static void sk_v4_out(const char *text, unsigned len)
 {
     unsigned i;
@@ -78,7 +82,7 @@ static unsigned sk_v4_readline(void)
     }
 }
 
-void sk_v4_run(void)
+void sk_v4_run(void (*chain)(void))
 {
     v4_boot boot;
 
@@ -86,10 +90,12 @@ void sk_v4_run(void)
     boot.n = &sk_v4_node; boot.es = &sk_v4_es; boot.h = &sk_v4_heat; boot.im = &v4_capsule_image;
     boot.out = sk_v4_out; boot.key = sk_v4_key;
     boot.words = sk_v4_words; boot.word_count = sizeof sk_v4_words / sizeof sk_v4_words[0]; boot.serve = sk_v4_serve;
-    if (!v4_boot_run(&boot, sk_v4_disk, SK_V4_BLOCKS)) {
+    blk_subsys_init(sk_v4_post_blk_ram, sizeof(sk_v4_post_blk_ram));
+    if (!v4_boot_run(&boot)) {
         console_println("StarForth v4: not started");
         for (;;) { }
     }
+    if (chain) chain();
 
     /* The prompt is the host's: read a line, hand it to the node, say how it
      * ended. */