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>

Kconfig.kernel

@@ -13,6 +13,17 @@ config STARFORTH_ENABLE_VM
 	  no capsules, no "ok" REPL. Gates a large source-file selection
 	  block in kernel/Makefile, not just a handful of -D flags.
 
+config STARFORTH_V4
+	bool "Boot StarForth v4, the F18-derived engine, at a single prompt (STARFORTH_V4)"
+	default n
+	help
+	  Instead of the v3 VM and its fleet, the kernel starts one StarForth
+	  v4 host node after the M0-M6 hardware milestones: the golden model
+	  of the 32-opcode engine (v4/src) running the capsule image built
+	  from v4/capsule, with its console on the kernel's serial console.
+	  It reaches v4's "ok> " prompt and stays there. The v3 VM is still
+	  compiled in but is not started. See docs/v4.0.0/DECOMPOSITION.md.
+
 config PARITY_MODE
 	bool "Deterministic parity harness mode (PARITY_MODE)"
 	default n