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/vt100.h

@@ -110,6 +110,22 @@ void vt100_puts(const char *s);
  */
 void vt100_toggle_graphics(void);
 
+/**
+ * Draw a solid block cursor at the terminal's current position. Static,
+ * not blinking. Idempotent -- safe to call after every keystroke, since
+ * whatever gets typed next naturally overwrites it. No-op before TTF mode
+ * is active or while in graphics mode (vt100_toggle_graphics()).
+ */
+void vt100_draw_cursor(void);
+
+/**
+ * Erases whatever vt100_draw_cursor() last drew, restoring the cell to
+ * plain background. Call before moving away from a cursor-drawn cell
+ * without also drawing a character there (e.g. Enter/newline). No-op
+ * before TTF mode is active or while in graphics mode.
+ */
+void vt100_erase_cursor(void);
+
 /* -----------------------------------------------------------------------
  * Queries
  * --------------------------------------------------------------------- */