feat(v4.0.0): the text assembler lays down dictionary headers

'header NAME [flags]' in front of a word gives it a dictionary entry in
the layout capsule/dict.v4 uses, linked to the header before it, so the
words of a capsule are in the dictionary as soon as it is assembled.
The name is taken as it stands (it may be ( or + or ;) and the
assembler's own name for the code may differ.

Tested in the assembler's own test, and by FIND and the field words
running over assembler-made entries beside ones (HEADER) made.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/include/v4/text.h

@@ -42,6 +42,15 @@
  *                    may use other macros; it may not define a word, a label
  *                    or a macro.
  *
+ *   header NAME      a dictionary entry for NAME (capsule/dict.v4), 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
+ *                    characters.  Between it and the `:` may come the flags
+ *                    immediate, data, inline and compile-only.  The word after
+ *                    `:` is the assembler's name for the code, as always, and
+ *                    need not be the same:   header + inline  : PLUS  + ;
+ *
  * A name is looked up in this order: opcode, macro, number, constant, label
  * (for a branch target), word.  So an opcode's name cannot be redefined, and
  * the FORTH words that share one (`@`, `!`, `+`, ...) are written as their
@@ -99,6 +108,9 @@ typedef struct {
     char     pool[V4_TEXT_POOL];
     unsigned pool_used;
 
+    v4_cell  latest;                   /* xt of the newest header, 0 if none */
+    int      header_pending;           /* a header waits for its `:` */
+
     unsigned line;
     int      failed;
     char     message[224];
@@ -125,6 +137,10 @@ int v4_text_finish(v4_text *t);
 /* The address of the word NAME, or -1 if there is none. */
 v4_cell v4_text_word(const v4_text *t, const char *name);
 
+/* The xt of the newest `header`, 0 if there has been none: what the
+ * dictionary's LATEST starts as. */
+v4_cell v4_text_latest(const v4_text *t);
+
 /* The next free word address. */
 v4_cell v4_text_here(v4_text *t);