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/capsule/capsule_blocks.c

@@ -0,0 +1,29 @@
+/* capsule_blocks.c -- the block format of a .4th capsule payload.
+ * See capsule_blocks.h.  Nothing here uses the C library or a VM.
+ */
+#include "starkernel/capsule_blocks.h"
+
+int capsule_block_header(const uint8_t *p, const uint8_t *end,
+                         uint32_t *out_num, const uint8_t **out_after)
+{
+    const uint8_t *q;
+    uint32_t num = 0;
+
+    if ((uint64_t)(end - p) < 7u) return 0;
+    if (p[0] != 'B' || p[1] != 'l' || p[2] != 'o' || p[3] != 'c' || p[4] != 'k' || p[5] != ' ')
+        return 0;
+
+    q = p + 6;
+    if (q >= end || *q < '0' || *q > '9') return 0;
+    while (q < end && *q >= '0' && *q <= '9') {
+        num = num * 10u + (uint32_t)(*q - '0');
+        q++;
+    }
+
+    while (q < end && *q != '\n') q++;
+    if (q < end) q++;
+
+    *out_num   = num;
+    *out_after = q;
+    return 1;
+}