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/include/v4/text.h

@@ -56,7 +56,21 @@
  *                    count byte, then its characters, four to a cell, low
  *                    byte first, zero-padded to a whole cell.
  *
- *   header NAME      a dictionary entry for NAME (capsule/dict.v4), laid down
+ *   set NAME VALUE   an assembler variable: like a constant, but it may be
+ *                    set again (the newest value is its value).  In code it
+ *                    is a literal.
+ *   here             as a VALUE: the next word address, the word being built
+ *                    ended first.  Not something to assemble on its own.
+ *   store ADDR VALUE ADDR, a VALUE that is a word of RAM, is given VALUE: the
+ *                    state the system is switched on in.
+ *   start NAME       the end of the text: every branch must have its target,
+ *                    and NAME, a word, is where the system starts
+ *                    (v4_text_start).  Nothing may follow.
+ *                    These four, constant, #, and ", are the ROM's language
+ *                    (docs/v4.0.0/MACHINE.md): the ROM assembles the same
+ *                    text the same way.
+ *
+ *   header NAME      a dictionary entry for NAME (capsules/v4/sys/dict.f18), laid down
  *                    in front of the word that the next `:` starts, and
  *                    linked to the header before it.  NAME is taken as it
  *                    stands -- it may be ( or \ or + -- and is at most 31
@@ -123,6 +137,7 @@ typedef struct {
     unsigned pool_used;
 
     v4_cell  latest;                   /* xt of the newest header, 0 if none */
+    v4_cell  start;                    /* the word `start` named, -1 if none yet */
     int      header_pending;           /* a header waits for its `:` */
 
     unsigned line;
@@ -155,6 +170,13 @@ v4_cell v4_text_word(const v4_text *t, const char *name);
  * dictionary's LATEST starts as. */
 v4_cell v4_text_latest(const v4_text *t);
 
+/* The value of the constant or set variable NAME: 1 and *value, or 0 if
+ * NAME is neither. */
+int v4_text_value(const v4_text *t, const char *name, v4_cell *value);
+
+/* The word `start` named, or -1 if the text has had no start. */
+v4_cell v4_text_start(const v4_text *t);
+
 /* The next free word address. */
 v4_cell v4_text_here(v4_text *t);