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_compile.c

@@ -1,7 +1,7 @@
 /* test_host_compile.c -- the defining and compiling words, executed on the
  * host node.
  *
- * capsule/compile.v4 and capsule/forth.v4 on top of the earlier layers: for
+ * capsules/v4/sys/compile.f18 and capsules/v4/sys/forth.f18 on top of the earlier layers: for
  * the first time FORTH source text goes in and running code comes out.  Each
  * test hands lines of source to INTERPRET, which defines words with : and ;
  * and the rest, runs them, and leaves results on the stack.
@@ -195,7 +195,7 @@ int main(void)
     printf("v4 host compiler tests: V4_CELL_BITS=%d, V4_NODE_WORDS=%u\n", V4_CELL_BITS, (unsigned)V4_NODE_WORDS);
 
     {
-        static const char *const files[] = { "core.v4", "input.v4", "dict.v4", "codegen.v4", "compile.v4", "quit.v4", "forth.v4" };
+        static const char *const files[] = { "core.f18", "input.f18", "dict.f18", "codegen.f18", "compile.f18", "quit.f18", "forth.f18" };
         CHECK(host_load(&tx, &n, files, 7), "the capsule assembles");
     }
     /* two in-line words of the test's own, to exercise the in-liner: one with