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>

include/starkernel/console.h

@@ -98,6 +98,20 @@ void console_fb_scroll_fwd(uint32_t n);
  */
 void console_fb_toggle_graphics(void);
 
+/**
+ * Thin wrapper over vt100_draw_cursor() -- see that function's doc
+ * comment for the full contract (a static block cursor at the terminal's
+ * current position). No-op if the framebuffer console was never
+ * initialized.
+ */
+void console_fb_draw_cursor(void);
+
+/**
+ * Thin wrapper over vt100_erase_cursor(). No-op if the framebuffer
+ * console was never initialized.
+ */
+void console_fb_erase_cursor(void);
+
 /**
  * Write a single character to serial console
  */