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>

capsules/v4/sys/system.f18

@@ -1,7 +1,7 @@
-\ system.v4 -- words about the dictionary as a whole: WORDS, FORGET, FENCE.
+\ system.f18 -- words about the dictionary as a whole: WORDS, FORGET, FENCE.
 \
 \ DECOMPOSITION.md 5.13 and 5.15.  Part of the compiler capsule; rests on
-\ dict.v4, quit.v4 and forth.v4.
+\ dict.f18, quit.f18 and forth.f18.
 \
 \ Constants the loader supplies:
 \   FENCE   word address of the variable: FORGET will not remove an entry
@@ -104,7 +104,7 @@ set HDR-NFA here  ", FORGET  #, 0  #, HDR-NFA  #, HDR-LINK  set HDR-LINK here
   C1: drop  (LATEST) CONTEXT a! !
   C2: CURRENT a! @ (Q) a! @ inv + 1 + -if C3  drop jump TOLD
   C3: drop  (LATEST) CURRENT a! !
-  TOLD: (Q) a! @  jump (FORGOTTEN)           \ the kernel's records of them go too (dict.v4)
+  TOLD: (Q) a! @  jump (FORGOTTEN)           \ the kernel's records of them go too (dict.f18)
   MISSING: drop  (UNKNOWN) drop ;
 
 \ ---- the system as a whole -------------------------------------------------------
@@ -143,7 +143,7 @@ set HDR-NFA here  ", COLD  #, 0  #, HDR-NFA  #, HDR-LINK  set HDR-LINK here
   (BOOT) a! @ 2/ 2/  FENCE a! !
   (LATEST) CONTEXT a! !  (LATEST) CURRENT a! !  0 VOC-LINK a! !
   10 BASE a! !  0 SCR a! !  2 (LOG-LEVEL) a! !  0 (ACL-HOOK) a! !  EMPTY-BUFFERS
-  (BOOT) a! @ 2/ 2/  (FORGOTTEN)             \ the kernel's records of what has gone go too (dict.v4)
+  (BOOT) a! @ 2/ 2/  (FORGOTTEN)             \ the kernel's records of what has gone go too (dict.f18)
   $54524F46 (EMIT4) $39372D48 (EMIT4) $6C6F4320 (EMIT4) $74532064 (EMIT4) $747261 (EMIT4) CR
   $74737953 (EMIT4) $69206D65 (EMIT4) $6974696E (EMIT4) $7A696C61 (EMIT4) $2E6465 (EMIT4) CR
   jump ABORT