feat(v4.0.0): a node is handed a line -- ENGINE.md step 1

A v4 node no longer reads its own command line or prints a prompt.  Its
host puts a line of text in the node's input buffer and starts it at
(LINE); the node interprets it and stops at (IDLE), leaving in
(LINE-STATUS) how it ended: completed, an error, or QUIT.  The host says
" ok" or " ERROR" and prompts, as the kernel's REPL does for a v3 VM.  A
line may be 1024 characters, a block, as v3's.  Ruled 2026-10-05
(V3-PARITY.md 1b); design ENGINE.md 3.1.

- quit.v4: (REPL), the node's prompt loop, is gone; (LINE) (IDLE) (DONE)
- image.h/.c: v4_line_begin, v4_line_done, v4_line_status; the node is
  idle at switch-on
- boot.c: v4_boot_line, the one loop the hosted binary, the kernel and the
  capsule loader hand a line with; the code that took " ok" and the prompt
  back out of the node's output is gone
- hosted.c, sk_v4.c: the prompt and the line editing are the host's
- test_host_quit.c: the tests are the node's host; two tests of the old
  80-character prompt line now test a whole line, 1024 and 1025 characters

Verified: make -C v4 test passes at both widths; hosted-check passes on
three ISAs; clean qemu with STARFORTH_V4=1 on amd64, aarch64 and riscv64
passes POST (550 of 550) with the same hashes as hosted, and three lines
typed at each bare-metal prompt through the serial port are answered
correctly (logs/20261005-180922, -181152, -181541).

Still the lone node: kernel_main.c starts it before the fleet tables.

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

v4/tools/hosted.c

@@ -8,7 +8,7 @@
  */
 #include "v4/boot.h"
 #include <stdio.h>
-#include <unistd.h>
+#include <string.h>
 
 #define BLOCKS 64u
 
@@ -22,27 +22,40 @@ static void console_out(const char *text, unsigned len)
     (void)fwrite(text, 1, len, stdout);
 }
 
+/* a character for a line that reads the keyboard */
+static int console_key(void)
+{
+    int c;
+    fflush(stdout);
+    c = getchar();
+    return c == EOF ? V4_BOOT_KEY_END : c;
+}
+
+/* The prompt is the host's, as it is the kernel's REPL's for a v3 VM: read a
+ * line, hand it to the node, say how it ended. */
 int main(void)
 {
-    const v4_image *im = &v4_capsule_image;
-    unsigned char buf[256];
+    static char line[1026];
     v4_boot boot;
 
-    boot.n = &n; boot.es = &es; boot.h = &h; boot.im = im; boot.out = console_out;
+    boot.n = &n; boot.es = &es; boot.h = &h; boot.im = &v4_capsule_image;
+    boot.out = console_out; boot.key = console_key;
     if (!v4_boot_run(&boot, disk, BLOCKS)) { fflush(stdout); return 1; }
+
     for (;;) {
-        (void)v4_exec_step_word(&n, &es, &h);
-        if (n.console_len) {
-            (void)fwrite(n.console, 1, n.console_len, stdout);
-            n.console_len = 0;
-        }
-        if (n.stopped) { fflush(stdout); fprintf(stderr, "starforth4: the node stopped on a fault\n"); return 1; }
-        if (v4_image_waiting(&n, im)) {
-            ssize_t got;
-            fflush(stdout);
-            got = read(0, buf, sizeof buf);
-            if (got <= 0) { putchar('\n'); return 0; }
-            (void)v4_node_console_feed(&n, buf, (unsigned)got);
-        }
+        size_t len;
+        int how;
+
+        fputs("ok> ", stdout);
+        fflush(stdout);
+        if (!fgets(line, sizeof line, stdin)) { putchar('\n'); return 0; }
+        len = strlen(line);
+        if (len && line[len - 1] == '\n') len--;
+        how = v4_boot_line(&boot, line, (unsigned)len);
+        if (how == V4_LINE_COMPLETED) fputs(" ok\n", stdout);
+        else if (how == V4_LINE_ERROR) fputs(" ERROR\n", stdout);
+        else if (how == V4_BOOT_LINE_TOO_LONG) fputs("Line too long\n ERROR\n", stdout);
+        else if (how == V4_BOOT_LINE_NO_INPUT) { putchar('\n'); return 0; }
+        else if (how == V4_BOOT_LINE_STOPPED) { fflush(stdout); fprintf(stderr, "starforth4: the node stopped on a fault\n"); return 1; }
     }
 }