feat(v4.0.0): CONSOLE-RX and CONSOLE-STATUS on the single-node model

The console's receive side, standing in for the console node until the
mesh exists, as CONSOLE-TX does for output.  A data fetch (@, @+, @b)
from CONSOLE-STATUS gives -1 when a character is pending and 0 when
not; from CONSOLE-RX it gives the next character and takes it, or -1
with none pending.  The characters come from a queue the test feeds.

Instruction words and literals are still fetched with v4_node_load, so
code at a register's address is never taken for the register.

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

v4/src/node.c

@@ -12,6 +12,7 @@ void v4_node_reset(v4_node *n)
     v4_dstack_reset(&n->ds);
     v4_rstack_reset(&n->rs);
     v4_node_console_attach(n, -1);
+    v4_node_console_input_attach(n, -1, -1);
 }
 
 int v4_node_guards_intact(const v4_node *n)
@@ -44,6 +45,45 @@ void v4_node_store(v4_node *n, v4_cell addr, v4_cell value)
     n->mem[(unsigned)addr] = value;
 }
 
+v4_cell v4_node_fetch(v4_node *n, v4_cell addr)
+{
+    /* console_rx and console_status are -1 when no input is attached, which
+     * no valid address equals. */
+    if (addr == n->console_status)
+        return n->input_pos < n->input_len ? (v4_cell)-1 : (v4_cell)0;
+    if (addr == n->console_rx) {
+        v4_cell c;
+        if (n->input_pos >= n->input_len) return (v4_cell)-1;
+        c = (v4_cell)n->input[n->input_pos++];
+        if (n->input_pos == n->input_len) n->input_pos = n->input_len = 0;
+        return c;
+    }
+    return v4_node_load(n, addr);
+}
+
+void v4_node_console_input_attach(v4_node *n, v4_cell rx, v4_cell status)
+{
+    n->console_rx = rx;
+    n->console_status = status;
+    n->input_len = 0;
+    n->input_pos = 0;
+}
+
+unsigned v4_node_console_feed(v4_node *n, const void *chars, unsigned len)
+{
+    const unsigned char *c = (const unsigned char *)chars;
+    unsigned i, pending = n->input_len - n->input_pos, took = 0;
+
+    /* Slide what is pending to the front so the whole buffer is usable. */
+    if (n->input_pos > 0) {
+        for (i = 0; i < pending; i++) n->input[i] = n->input[n->input_pos + i];
+        n->input_pos = 0;
+        n->input_len = pending;
+    }
+    while (took < len && n->input_len < V4_CONSOLE_CAP) n->input[n->input_len++] = c[took++];
+    return took;
+}
+
 void v4_node_console_attach(v4_node *n, v4_cell addr)
 {
     n->console_tx = addr;