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/input.f18

@@ -1,10 +1,10 @@
-\ input.v4 -- reading a line and splitting it into words and numbers.
+\ input.f18 -- reading a line and splitting it into words and numbers.
 \
 \ DECOMPOSITION.md 5.9: TIB >IN SPAN SOURCE BL EXPECT QUERY WORD ENCLOSE
 \ CONVERT NUMBER, and the comment words ( and \ of 5.15 (named PAREN and
 \ BACKSLASH here, since the text assembler reads those two characters as its
 \ own comments; the dictionary will give them their names).  Part of the compiler
-\ capsule: it runs on the host node.  Rests on core.v4.
+\ capsule: it runs on the host node.  Rests on core.f18.
 \
 \ Constants the loader supplies:
 \   TIB      byte address of the text input buffer; QUERY fills at most 81 bytes
@@ -14,7 +14,7 @@
 \   WBUF     byte address of WORD's buffer, 257 bytes
 \   (P)      word address of eight cells of scratch for this file
 \   (SRC)    word address of the variable: the byte address of the text being
-\            interpreted.  QUERY makes it TIB; LOAD (blocks.v4) makes it a
+\            interpreted.  QUERY makes it TIB; LOAD (blocks.f18) makes it a
 \            block buffer.  >IN is an offset from it and SPAN its length.
 \   (SRC-HOOK)   word address of a variable: 0, or the address of a word
 \            that WORD calls before it looks at the text, to make sure (SRC)