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/tools/mkmap.c

@@ -2,6 +2,7 @@
  *
  *     mkmap > rom-map-<width>.f18
  *     mkmap sys > capsules/v4/sys/map-<width>.f18
+ *     mkmap image > v4_map_<width>.c
  *
  * Runs on the build machine, built for the host node (HOST_WORDS and the
  * host stack sizes) at one cell width.  It writes, as `constant` lines in
@@ -33,11 +34,30 @@ static int system_map(void)
     return 0;
 }
 
+/* `mkmap image`: the system as its host knows it before it is built
+ * (include/v4/image.h, v4_map_image): what the build was for, the map's
+ * addresses, and where the image record and the ROM's error are.  What only
+ * the assembled system knows -- where it waits, its fault table, KEY -- is
+ * -1 here; v4_image_from_node reads it from the image record. */
+static int image_description(void)
+{
+    printf("/* Generated by v4/tools/mkmap.c (mkmap image) from v4/tests/host_map.h -- do not edit. */\n");
+    printf("#include \"v4/image.h\"\n\nconst v4_image v4_map_image = {\n");
+    printf("    %uu, %uu, %uu, %uu,\n", (unsigned)V4_CELL_BITS, (unsigned)V4_NODE_WORDS, (unsigned)V4_DATA_RING, (unsigned)V4_RET_RING);
+    printf("    -1,\n    -1, %ld,\n    -1, -1,\n", (long)PORT);
+    printf("    %ld, %ld, %ld,\n    %ld, %ld,\n    %ld,\n    %ld, %ld,\n    %ld, %ld,\n    %ld,\n",
+           (long)CONSOLE_TX, (long)CONSOLE_RX, (long)CONSOLE_ST, (long)DSTACK_REG, (long)RSTACK_REG,
+           (long)NODE_ERROR, (long)DP, (long)LATEST, (long)BOOT_CELLS, (long)FENCE, (long)BUF0_W);
+    printf("    %ld, %ld, %ld\n};\n", (long)ROM_IMG, (long)ROM_FAILED, (long)ROM_ERRLINE);
+    return 0;
+}
+
 int main(int argc, char **argv)
 {
     (void)host_load;                         /* host_map.h's loader is not this tool's */
     (void)host_branch_slots;
     if (argc == 2 && argv[1][0] == 's') return system_map();
+    if (argc == 2 && argv[1][0] == 'i') return image_description();
     printf("\\ Generated by v4/tools/mkmap.c from v4/tests/host_map.h -- do not edit.\n");
     constant("CONSOLE-RX", (long)CONSOLE_RX);
     constant("CONSOLE-STATUS", (long)CONSOLE_ST);