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/include/starkernel/capsule_blocks.h

@@ -0,0 +1,21 @@
+/* capsule_blocks.h -- the block format of a .4th capsule payload.
+ *
+ * A .4th capsule is text: "Block <num>" header lines, each followed by that
+ * block's content lines (tools/mkcapsule.c, validate_forth_blocks).  This is
+ * the one place that says what a header line is.  It depends on no VM, so
+ * every loader of capsules can use it: the kernel's and the hosted v4
+ * system's (docs/v4.0.0/NUCLEUS.md 5.3).
+ */
+#ifndef STARKERNEL_CAPSULE_BLOCKS_H
+#define STARKERNEL_CAPSULE_BLOCKS_H
+
+#include <stdint.h>
+
+/* Is the line that starts at p a "Block <num>" header?  If so, returns 1
+ * with the number in *out_num and the start of the next line in *out_after;
+ * otherwise returns 0 and changes nothing.  `end` is one past the payload's
+ * last byte. */
+int capsule_block_header(const uint8_t *p, const uint8_t *end,
+                         uint32_t *out_num, const uint8_t **out_after);
+
+#endif /* STARKERNEL_CAPSULE_BLOCKS_H */

kernel/include/starkernel/v4/sk_v4.h

@@ -0,0 +1,13 @@
+/* sk_v4.h -- StarForth v4 on bare metal: one host node at a prompt.
+ *
+ * Built only with STARFORTH_V4=1 (Kconfig.kernel).
+ */
+#ifndef STARKERNEL_V4_SK_V4_H
+#define STARKERNEL_V4_SK_V4_H
+
+/* Start one StarForth v4 host node from the capsule image linked into the
+ * kernel, with its console on the kernel's console, and run it.  Does not
+ * return. */
+void sk_v4_run(void);
+
+#endif /* STARKERNEL_V4_SK_V4_H */