feat(v4.0.0): the system boots from a nucleus and loads its capsules

One boot, v4/system/boot.c, for both products: it starts the nucleus image,
finds each capsule in the baked capsule directory, recomputes its hash,
checks its signature, gives its blocks to the node a line at a time, and
prints PARITY:V4_NUCLEUS, PARITY:V4_CAPSULE and PARITY:OK before the prompt.
A line the node does not accept ends the boot with the capsule, block and
line named.  docs/v4.0.0/NUCLEUS.md.

- hosted Linux product for amd64, aarch64 and riscv64 (make -C v4 hosted);
  make -C v4 hosted-check boots all three and requires identical output
- the kernel's v4 entry (STARFORTH_V4=1) calls the same boot
- capsules/v4/forth79.4th, block 6000: no definitions yet
- mkimage builds the nucleus only; no FORTH source is compiled at build time
- capsule_blocks.c: the Block-header parse, free of any VM, for every loader

Verified: make -C v4 test passes; hosted-check passes on the three ISAs with
the same hashes; the kernel compiles with STARFORTH_V4=1 on the three.
Not verified: no bare-metal boot of v4 has been run.

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

kernel/src/kernel_main.c

@@ -81,6 +81,9 @@ EFI_RUNTIME_SERVICES *g_sk_runtime_services = NULL;
                             log.h's line length (LOG_MSG_LINE_MAX, 256)
                             are distinct names */
 #include "version.h"
+#ifdef STARFORTH_V4
+#include "starkernel/v4/sk_v4.h"
+#endif
 #endif
 
 /* Forward declaration — kernel_main_deep contains everything from heartbeat
@@ -512,6 +515,14 @@ static void kernel_main_deep(BootInfo *boot_info) {
     console_println("Kernel initialization complete.");
     console_println("Boot successful!\n");
 
+#ifdef STARFORTH_V4
+    /* StarForth v4 at a single prompt (Kconfig STARFORTH_V4): one host node
+     * of the F18-derived engine, in place of the v3 VM and everything below.
+     * It does not return. */
+    (void)boot_info;
+    sk_v4_run();
+#endif
+
 #ifdef STARFORTH_ENABLE_VM
     /* Stadium: boot-time allocation (FABRIC-0.md item 3.2), before any VM
      * exists (§6). Soft failure -- nothing downstream consumes the Stadium