feat(v4): the machine boots from its ROM; the prebuilt nucleus is gone

The host node starts in its ROM (entry 12), and the boot feeds it the
system's source through the console: capsules/v4/sys/map-64.f18, the
fourteen .f18 blobs and end.f18, each found, hash-checked and
signature-checked as any capsule, then EOT.  The ROM assembles the system
and starts it at (IDLE); the host reads the image record
(v4_image_from_node) for what only the assembled system knows.  A ROM
error is reported with its blob, line and meaning, then PARITY:FAIL.

Deleted: v4/tools/mkimage.c, capsules/v4/nucleus-64.f18, v4/capsule/*.v4,
v4/tools/v4tof18.py.  The .f18 files are the source now.

- text.c speaks the rest of the ROM's language (set, here, store, start;
  v4_text_value, v4_text_start), so the oracle reads the same source:
  test_host_rom_nucleus compares the ROM against text.c on the .f18 files,
  word for word, image record included.  Host tests load the .f18 files.
- mkmap image writes v4_map_image (the map's description of the system);
  hosted and the kernel link the ROM and the map instead of v4_image_64.c.
- Hera (Captain Bob's ruling, option 1): births stream the system the ROM
  assembled, as assembled code; hera.4th is unchanged.
  test_host_unit's host builds the system with the ROM.
- New parity lines: PARITY:V4_F18 per blob, PARITY:V4_ROM words=2049
  rom_hash=0xa6c792b5821010b7 instructions=60725216.

Acceptance: amd64, aarch64, riscv64 boot to ok> with PARITY:V4_POST
tests=538 pass=538 fail=0, POST: PASSED, and PARITY:V4_SYSTEM
word_count=317 dict_hash=0xe5a57f3eb72ff94a on all three -- the same hash
as the prebuilt nucleus gave.  make -C v4 test test-32 hosted-check: all
pass.  Logs: logs/20261015-141648, -141938, -142350.

Startup cost under QEMU TCG, the ROM's assembly (last V4_F18 line to
V4_ROM): amd64 26.7 s of 132 s to ok>; aarch64 41.0 s of 195 s; riscv64
20.5 s of 101 s.  Hosted native: +0.85 s.

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

kernel/src/v4/sk_v4.c

@@ -2,8 +2,9 @@
  *
  * The node is the golden model of the F18-derived engine (v4/src).  It comes
  * up as the hosted v4 system does, by v4_boot_run (v4/include/v4/boot.h):
- * the nucleus image, then the capsules from the directory baked into this
- * kernel, each checked and its parity line printed, then the prompt.
+ * the machine's ROM assembles the system from its source, then the capsules
+ * from the directory baked into this kernel are loaded, each checked and its
+ * parity line printed, then the prompt.
  * docs/v4.0.0/NUCLEUS.md.  This file is all the kernel adds:
  *
  *   - what the node prints goes to the kernel's console;
@@ -27,6 +28,7 @@
 static v4_node       sk_v4_node;
 static v4_exec_state sk_v4_es;
 static v4_heat       sk_v4_heat;
+static v4_image      sk_v4_image;
 
 static char          sk_v4_text[SK_V4_LINE + 1u];
 
@@ -87,7 +89,8 @@ void sk_v4_run(void (*chain)(void))
     v4_boot boot;
 
     console_println("StarForth v4: one host node, the F18-derived engine");
-    boot.n = &sk_v4_node; boot.es = &sk_v4_es; boot.h = &sk_v4_heat; boot.im = &v4_capsule_image;
+    sk_v4_image = v4_map_image;
+    boot.n = &sk_v4_node; boot.es = &sk_v4_es; boot.h = &sk_v4_heat; boot.im = &sk_v4_image;
     boot.out = sk_v4_out; boot.key = sk_v4_key;
     boot.words = sk_v4_words; boot.word_count = sizeof sk_v4_words / sizeof sk_v4_words[0]; boot.serve = sk_v4_serve;
     blk_subsys_init(sk_v4_post_blk_ram, sizeof(sk_v4_post_blk_ram));