feat(v4.0.0): the nucleus is a capsule, sent to a node born empty

MESH.md step 2.  A capsule of F18 code is the words a neighbour writes to a
node's port: for each stretch of memory, "@p a! @p push", the address and
count, "@p !+ unext" and the words; then a jump to the start.  A node born
empty executes that from its port, so it needs nothing in it beforehand.

- capsule.h/.c: v4_capsule_write, any node's memory as such a capsule
- mkimage writes the nucleus so, to capsules/v4/nucleus-64.f18, and the
  addresses a host needs as a C file; the memory image is no longer linked
  into either product
- mkcapsule is unchanged: the nucleus capsule is a built file kept under
  capsules/, as BLOCK_MAP.md is, and is baked, hashed and signed with the
  rest
- boot: the node is born empty (v4_image_born); the nucleus capsule is
  found, its hash and signature checked, and given to the node a word at a
  time as it reads its port; PARITY:V4_NUCLEUS carries its name and hash

Verified: test_fabric.c (59 checks, both widths, and under ASan and UBSan):
a memory with a programme and scattered words arrives word for word in an
empty node and runs.  The nucleus capsule rebuilds byte for byte.
hosted-check passes on three ISAs; clean qemu with STARFORTH_V4=1 on amd64,
aarch64 and riscv64 takes the nucleus in, passes POST (550 of 550) and
answers lines typed at each prompt (logs/20261006-102421, -102706,
-103048).

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

v4/README.md

@@ -29,7 +29,7 @@ under ASan and UBSan. There is no POST and no K measurement yet.
 | Part | Where | What it is |
 |---|---|---|
 | Engine | `v4/src` | The golden model of the 32-opcode node |
-| Nucleus | `v4/capsule/*.v4`, built by `v4/tools/mkimage.c` | The assembled words, as a memory image linked into the binary |
+| Nucleus | `v4/capsule/*.v4`, built by `v4/tools/mkimage.c` into `capsules/v4/nucleus-64.f18` | The assembled words, as a capsule of F18 code: sent through its port to a node born empty |
 | Capsules | `capsules/v4/*.4th`, baked by `tools/mkcapsule.c` | FORTH source, loaded when the system comes up |
 | Boot | `v4/system/boot.c` | Starts the nucleus, checks and loads each capsule, prints the parity lines, gives the prompt |
 
@@ -59,7 +59,7 @@ State, 2026-10-05, after `ENGINE.md` step 1: all six builds start the nucleus,
 load `v4:forth79.4th`, pass POST and reach `ok>`, with the same lines:
 
 ```
-PARITY:V4_NUCLEUS words=298 image_hash=0xd982cf093aadb8b9
+PARITY:V4_NUCLEUS name=v4:nucleus-64.f18 capsule_id=... capsule_hash=0xbcc90cc5a638a299 words=298
 PARITY:V4_CAPSULE name=v4:forth79.4th capsule_id=... dict_hash=...
 PARITY:V4_POST tests=550 pass=550 fail=0
 PARITY:V4_CAPSULE name=v4:post79.4th capsule_id=... dict_hash=...
@@ -68,8 +68,8 @@ POST: PASSED
 ok>
 ```
 
-Bare-metal logs: `logs/20261006-074907/amd64/`, `logs/20261006-075150/aarch64/`,
-`logs/20261006-075532/riscv64/`. On each, seven lines were then typed at the
+Bare-metal logs: `logs/20261006-102421/amd64/`, `logs/20261006-102706/aarch64/`,
+`logs/20261006-103048/riscv64/`. On each, seven lines were then typed at the
 prompt through the serial port — a definition, its use with the capsule's
 `U*`, `COLD`, the capsule word again, a `FORGET` of it (refused), a kernel
 word no one serves, and `BYE` — and each was answered as the hosted binary
@@ -105,7 +105,8 @@ everything about word records, the fleet and identity is its alone.
 making v4 equal v3 on bare metal. Its first part is built and tested in
 the engine: a node has `V4_PORTS` ports; a read from one blocks until the
 neighbour writes and a write until the neighbour reads; a node is born
-empty with `P` at its ports and executes what a neighbour sends it; and
+empty with `P` at its ports and executes what a neighbour sends it, which
+is how each product's node now gets its nucleus, as a capsule; and
 the fabric (`v4/src/fabric.c`) is the nodes there are and the table of how
 their ports are wired, both of which change while they run. The products
 below are still one node each; they become the five nodes at `MESH.md`