Cursor indicator + HB-ON/HB-OFF runtime DoE instrumentation toggle

Cursor (Captain Bob: "the only thing we need is a cursor"):
vt100_draw_cursor() draws a solid block at the terminal's current
position, called from repl.c after the prompt prints and after every
keystroke/backspace. vt100_erase_cursor() cleans up the one gap a static
cursor has -- Enter/newline moves away from the cursor cell without a
character draw ever overwriting it, which left a stray block behind
until this fix.

HB-ON/HB-OFF (Captain Bob: run a program with or without instrumentation
without rebuilding):
Converted per-tick DoE logging from a build-time flag (HEARTBEAT_DOE_LOG)
to a runtime one. doe_log_tick_row() now self-gates on g_doe_log_enabled
(default 1, matching the old default) instead of being compiled out
entirely; the call site in vm_runtime.c is unconditional. Two new FORTH
words, HB-ON and HB-OFF, flip the flag live. Removed the now-dead
HEARTBEAT_DOE_LOG plumbing: the Kconfig symbol, and the -D forwarding in
both LOADER_CFLAGS and KERNEL_CFLAGS.

Verified: three-arch clean QEMU boot + logs; dictionary word count 466
(463 baseline + ALT+TAB + HB-ON + HB-OFF, exactly the three words added
across this session); amd64 screendump confirms the cursor renders
correctly after real interactive typing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/starkernel/repl.c

@@ -190,6 +190,7 @@ static int sk_readline(char *buf, int size)
     int n = 0;
 
     buf[0] = '\0';
+    console_fb_draw_cursor();   /* show the cursor at the bare prompt, before any input */
 
     for (;;) {
         int c = console_getc();   /* non-blocking poll */
@@ -223,6 +224,7 @@ static int sk_readline(char *buf, int size)
         }
 
         if (c == '\r' || c == '\n') {
+            console_fb_erase_cursor();   /* leaving this cell without drawing a char over it */
             console_putc('\n');
             break;
         }
@@ -235,6 +237,7 @@ static int sk_readline(char *buf, int size)
             console_putc('\b');
             console_putc(' ');
             console_putc('\b');
+            console_fb_draw_cursor();
             continue;
         }
 
@@ -244,6 +247,7 @@ static int sk_readline(char *buf, int size)
         buf[n++] = (char)c;
         buf[n] = '\0';
         console_putc((char)c);        /* echo */
+        console_fb_draw_cursor();
     }
 
     buf[n] = '\0';