FABRIC-2.md §I.9 follow-on: blinking | cursor instead of static block

Captain Bob asked for the framebuffer cursor to render as a blinking
vertical bar rather than the previous static solid-block glyph.

vt100_draw_cursor() (hal/vt100.c) now fills a thin bar (cell_w()/8, min
1px, full cell height) at the cursor's left edge instead of the whole
cell -- an I-beam shape. vt100_erase_cursor() is unchanged (clearing the
whole cell already safely covers the narrower bar).

Blinking is new in repl.c: sk_console_readline()'s idle branch toggles the
cursor on/off every SK_CURSOR_BLINK_INTERVAL (50 ticks, 500ms at 100Hz)
via alternating console_fb_draw_cursor()/console_fb_erase_cursor() calls,
independent of the heartbeat/idle-beat mechanism the §I.9 fix just touched
(deliberately not reused, to avoid recoupling to that path). Runs
regardless of n, so it blinks whether sitting at a bare prompt or paused
mid-edit. Every deterministic draw site (initial prompt, prompt reanchor,
backspace, character echo) now goes through a new helper, sk_cursor_show(),
which resets the blink cycle to "on" and redraws -- typing always shows a
solid cursor, never mid-blink.

Verified via the mandatory foreground 3-arch QEMU acceptance boot: amd64
(logs/20260905-021054, extensive live interactive typing including
multi-line : / ; word definitions and error cases, prompts stayed
correctly attached throughout), aarch64 (logs/20260905-021551), riscv64
(logs/20260905-022324) -- all three reached (zuse) ok> and shut down
cleanly via BYE.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

src/starkernel/hal/vt100.c

@@ -354,20 +354,24 @@ static void draw_cursor_glyph(uint8_t ch)
     }
 }
 
-/* Solid block cursor at the current position (g_vt.cx, g_vt.cy). Not
- * blinking -- a static indicator, simplest thing that actually shows
- * where typed input lands, callable as often as the caller likes since
- * it is idempotent (always draws the same thing at the current position).
- * A character typed at this position naturally overwrites it --
- * draw_cursor_glyph() always fills the whole cell with bg before drawing
- * a glyph -- but *leaving* this position without drawing a character
- * (Enter/newline) does not, which is what vt100_erase_cursor() below is
- * for. No-op outside TTF mode or graphics mode (4.4ab's
+/* Thin vertical-bar ("I-beam") cursor at the current position (g_vt.cx,
+ * g_vt.cy) -- 2026-09-05, replaces the earlier solid block glyph per
+ * request. Idempotent (always draws the same bar at the current
+ * position), so callable as often as the caller likes; blinking (repl.c,
+ * sk_console_readline()'s idle loop) is layered on top by alternating
+ * calls to this and vt100_erase_cursor() on a timer, not by anything in
+ * here. A character typed at this position naturally overwrites the bar
+ * -- draw_cursor_glyph() always fills the whole cell with bg before
+ * drawing a glyph -- but *leaving* this position without drawing a
+ * character (Enter/newline) does not, which is what vt100_erase_cursor()
+ * below is for. No-op outside TTF mode or graphics mode (4.4ab's
  * g_terminal_visible), matching every other draw call in this file. */
 void vt100_draw_cursor(void)
 {
     if (!g_terminal_visible || g_glyph_mode != VT_GLYPH_TTF) return;
-    fb_fill_rect(px_of(g_vt.cx), py_of(g_vt.cy), cell_w(), cell_h(), eff_fg());
+    uint32_t bar_w = cell_w() / 8u;
+    if (bar_w < 1u) bar_w = 1u;
+    fb_fill_rect(px_of(g_vt.cx), py_of(g_vt.cy), bar_w, cell_h(), eff_fg());
 }
 
 /* Erases whatever vt100_draw_cursor() last drew at the current position,

src/starkernel/repl.c

@@ -144,6 +144,28 @@ blkio_dev_t *sk_repl_get_attached_blk_dev(void) {
 
 static uint64_t g_last_beat_tick;    /* zero-initialized (BSS) */
 
+/* Cursor blink, 2026-09-05: the framebuffer cursor (now a thin vertical
+ * bar, vt100.c's vt100_draw_cursor()) blinks on/off every
+ * SK_CURSOR_BLINK_INTERVAL ticks while sk_console_readline()'s idle loop
+ * is spinning -- i.e. whenever nothing has been typed for that long,
+ * whether sitting at a bare prompt or paused mid-edit. g_cursor_visible
+ * tracks which half of the blink cycle is current; sk_cursor_show() below
+ * is the single place that resets the cycle back to "on" and redraws --
+ * every deterministic draw site (fresh prompt, echoed character,
+ * backspace) calls it instead of vt100_draw_cursor() directly, so typing
+ * always shows a solid cursor rather than possibly landing mid-blink. */
+#define SK_CURSOR_BLINK_INTERVAL  50u   /* ticks between blink toggles (500 ms at 100 Hz) */
+
+static uint64_t g_cursor_blink_tick; /* zero-initialized (BSS) */
+static int      g_cursor_visible = 1;
+
+static void sk_cursor_show(void)
+{
+    g_cursor_visible    = 1;
+    g_cursor_blink_tick = heartbeat_ticks();
+    console_fb_draw_cursor();
+}
+
 /* Reentrancy guards for the MSG-TICK pump inside sk_repl_idle().
  *
  * sk_repl_idle() runs vm_interpret(mama, ...) (below) to VM-EXEC MSG-TICK
@@ -591,7 +613,7 @@ int sk_console_readline(char* buf, int size, VM* active_vm, int reanchor_prompt)
     uint64_t prompt_tx_mark = console_tx_count();
 
     buf[0] = '\0';
-    console_fb_draw_cursor();   /* show the cursor at the bare prompt, before any input */
+    sk_cursor_show();   /* show the cursor at the bare prompt, before any input */
 
     for (;;) {
         int c;
@@ -637,6 +659,19 @@ int sk_console_readline(char* buf, int size, VM* active_vm, int reanchor_prompt)
                 sk_repl_idle(active_vm);
             }
 
+            /* Blink the cursor while idle (no key ready this iteration),
+             * regardless of n -- a real terminal blinks whether sitting at
+             * a bare prompt or paused mid-edit. sk_cursor_show() (called
+             * from every deterministic draw site below and at entry) resets
+             * this cycle to "on" on every real keystroke, so typing never
+             * looks like it landed mid-blink. */
+            if (now - g_cursor_blink_tick >= SK_CURSOR_BLINK_INTERVAL) {
+                g_cursor_blink_tick = now;
+                g_cursor_visible = !g_cursor_visible;
+                if (g_cursor_visible) console_fb_draw_cursor();
+                else                  console_fb_erase_cursor();
+            }
+
             /*
              * Re-anchor the prompt (FABRIC-0.md 4.4a unified prompt: print
              * only "ok> " here -- console_putc() auto-prefixes the current
@@ -659,7 +694,7 @@ int sk_console_readline(char* buf, int size, VM* active_vm, int reanchor_prompt)
                 console_tx_count() != prompt_tx_mark)
             {
                 sk_print_prompt();
-                console_fb_draw_cursor();
+                sk_cursor_show();
                 prompt_tx_mark = console_tx_count();
             }
 
@@ -689,7 +724,7 @@ int sk_console_readline(char* buf, int size, VM* active_vm, int reanchor_prompt)
             console_putc('\b');
             console_putc(' ');
             console_putc('\b');
-            console_fb_draw_cursor();
+            sk_cursor_show();
             continue;
         }
 
@@ -699,7 +734,7 @@ int sk_console_readline(char* buf, int size, VM* active_vm, int reanchor_prompt)
         buf[n++] = (char)c;
         buf[n] = '\0';
         console_putc((char)c);        /* echo */
-        console_fb_draw_cursor();
+        sk_cursor_show();
     }
 
     buf[n] = '\0';