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/tests/host_map.h

@@ -51,7 +51,7 @@
 #define CFS_CELLS    32
 #define WBUF_W       (TOP - 170)                /* WORD's buffer: 65 cells = 260 bytes */
 #define WBUF_CELLS   65
-#define TIB_W        (TOP - 440)                /* the text input buffer: 260 cells = 1040 bytes */
+#define TIB_W        (TOP - 440)                /* the text input buffer: 260 cells = 1040 bytes; a line is at most 1024 */
 #define PAD_W        (TOP - 470)                /* PAD: 21 cells = 84 bytes */
 #define SBUF_W       (TOP - 540)                /* 64 cells for the tests' own strings */
 #define HBUF_W       (SVARS - 16)               /* the hold buffer: 16 cells = 64 bytes */
@@ -67,6 +67,7 @@
 #define ACL_HOOK     (BUF0_W - 6)               /* the xt of the access control recheck word, or 0 */
 #define EMIT_HOOK    (BUF0_W - 7)               /* the xt of a word EMIT gives its character to, or 0: the console */
 #define CATCH        (BUF0_W - 8)               /* non-zero: a line's error is recorded here, -1, and the line ends " ok" */
+#define LINE_STATUS  (BUF0_W - 9)               /* how the last line ended: 0 QUIT, 1 completed, 2 an error */
 #define LOG_LEVEL    (BUF0_W - 5)               /* the level at or below which a message is printed */
 #define BOOT_CELLS   (BVARS + 14)               /* (BOOT): DP and LATEST as the loader left them, 2 cells */
 #define BUF0_W       (BVARS - 2 * 256)          /* the two block buffers, 256 cells each */
@@ -142,6 +143,7 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
     v4_text_constant(tx, "(SRC-HOOK)", SRC_HOOK);
     v4_text_constant(tx, "(EMIT-HOOK)", EMIT_HOOK);
     v4_text_constant(tx, "(CATCH)", CATCH);
+    v4_text_constant(tx, "(LINE-STATUS)", LINE_STATUS);
     v4_text_constant(tx, "BLOCK-NUMBER", STORAGE_REG);
     v4_text_constant(tx, "BLOCK-ADDRESS", STORAGE_REG + 1);
     v4_text_constant(tx, "BLOCK-COMMAND", STORAGE_REG + 2);