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>

v4/tests/test_host_codegen.c

@@ -1,6 +1,6 @@
 /* test_host_codegen.c -- the code generator, executed on the host node.
  *
- * capsule/codegen.v4 packs opcodes, literals and branches into instruction
+ * capsules/v4/sys/codegen.f18 packs opcodes, literals and branches into instruction
  * words at HERE; it is what the compiling words will be built on.  It is the
  * third thing in this tree that does that job -- after the slot packer in C
  * (asm.h) and the text assembler on top of it (text.h) -- and the first that
@@ -107,7 +107,7 @@ static int lay_by_text(void)
     return v4_text_assemble(&rtx, src) && v4_text_finish(&rtx);
 }
 
-/* By the code generator, on `n`: one call to a word of codegen.v4 for each
+/* By the code generator, on `n`: one call to a word of codegen.f18 for each
  * item, as the compiling words will make them. */
 static int lay_by_generator(void)
 {
@@ -179,7 +179,7 @@ int main(void)
     CHECK(bslots == 3, "at this node size a branch may sit in slots 0 .. 2: %u", bslots);
 
     {
-        static const char *const files[] = { "core.v4", "input.v4", "dict.v4", "codegen.v4" };
+        static const char *const files[] = { "core.f18", "input.f18", "dict.f18", "codegen.f18" };
         CHECK(host_load(&tx, &n, files, 4), "the capsule assembles");
     }
     CHECK(v4_text_finish(&tx), "everything is defined: %s", v4_text_error(&tx));