sk_repl_idle() auto-flush: implement Section V's "anything dirty? no? done" check

Makes blk_vm_flush_all() (block_words.c) non-static and declares it in
block_words.h -- it's already the entire implementation behind
SAVE-BUFFERS (block_word_save_buffers() is a one-line wrapper), so
sk_repl_idle() can call the exact same flush path outside word dispatch
without duplicating any logic. Cheap every idle tick regardless of dirty
state: every check inside is a small fixed-size scan, so no separate
pre-check was needed on top of it.

Caught a real bug via a live persistence test before trusting the
feature: the first version gated the flush on sk_repl_get_active_vm()
returning non-NULL, but NULL is that accessor's documented default
(Tripod's own USE-redirect override, "restore default dispatch") --
without an active USE redirect, the flush silently no-op'd for the
entire session. Confirmed live: wrote a byte via BUFFER (no
UPDATE/SAVE-BUFFERS), waited past the idle cadence, killed QEMU abruptly,
rebooted with the same disk image, read back 0 instead of the written
65. Fixed by threading the VM sk_repl_run()'s own loop already resolves
each iteration (g_repl_active_vm ? g_repl_active_vm : vm) down as a
parameter through sk_readline() into sk_repl_idle(), rather than trying
to re-derive it from an accessor with the wrong default. Re-ran the same
test after the fix: read back 65, matching the written byte -- the write
survived an abrupt kill with no explicit flush call anywhere in the
test, proving the idle-tick auto-flush genuinely ran.

All three architectures re-verified clean. FABRIC-2.md Section V item 6
and the corresponding Milestone 3 punch-list item marked done.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXjAPTEKrgY2Mrk25KoLDn

src/starkernel/repl.c

@@ -38,6 +38,7 @@
 #include "starkernel/blkio_usb.h"
 #include "block_subsystem.h"
 #include "word_source/include/keyboard_words.h"
+#include "word_source/include/block_words.h"
 #include <stdint.h>
 #include <string.h>
 
@@ -77,7 +78,7 @@ VM  *sk_repl_get_active_vm(void)   { return g_repl_active_vm; }
 
 static uint64_t g_last_beat_tick;    /* zero-initialized (BSS) */
 
-static void sk_repl_idle(void)
+static void sk_repl_idle(VM *active_vm)
 {
     /* Artemis Milestone 2d: xHCI Event Ring servicing. This is exactly the
      * "interrupt-driven, coarse cadence, cheap early-exit" trigger Section
@@ -124,6 +125,28 @@ static void sk_repl_idle(void)
         xdev->bot_msc_detach_pending = 0;
         blk_subsys_detach_device(&usb_blk_dev);
     }
+
+    /* FABRIC.md/FABRIC-2.md Section V item 6: "a cheap 'anything dirty?
+     * no? done' block-sync check", the same "interrupt-driven, coarse
+     * cadence, cheap early-exit" trigger shape as the xHCI servicing
+     * above -- this was the one piece of that design already fully
+     * specified and waiting for this hook to actually be non-empty.
+     * blk_vm_flush_all() (block_words.c, the same code SAVE-BUFFERS
+     * itself runs) is cheap to call when nothing is dirty -- every
+     * check inside is a small fixed-size scan, no disk I/O happens
+     * unless something genuinely needs writing -- so no separate
+     * "is anything dirty" pre-check is needed here.
+     *
+     * active_vm is passed in by the caller (sk_readline(), itself passed
+     * through from sk_repl_run()/sk_repl_step()'s own already-resolved
+     * VM) rather than read via sk_repl_get_active_vm() here -- that
+     * accessor returns NULL whenever Tripod's USE word hasn't redirected
+     * it, which is the common case, not "no VM is active." An earlier
+     * version of this code called sk_repl_get_active_vm() directly and
+     * silently no-op'd for exactly that reason, confirmed live: a BUFFER
+     * write with no UPDATE, followed by an idle wait and an abrupt kill,
+     * did not survive a reboot until this fix. */
+    blk_vm_flush_all(active_vm);
 }
 
 /*===========================================================================
@@ -231,7 +254,7 @@ static int sk_kbd_getc(void)
  * Returns the number of characters placed in buf (not counting '\0').
  *===========================================================================*/
 
-static int sk_readline(char *buf, int size)
+static int sk_readline(char *buf, int size, VM *active_vm)
 {
     int n = 0;
 
@@ -255,7 +278,7 @@ static int sk_readline(char *buf, int size)
             uint64_t now = heartbeat_ticks();
             if (now - g_last_beat_tick >= SK_IDLE_BEAT_INTERVAL) {
                 g_last_beat_tick = now;
-                sk_repl_idle();
+                sk_repl_idle(active_vm);
             }
             /*
              * Do NOT use hlt here: QEMU single-threaded TCG can't process
@@ -350,7 +373,7 @@ int sk_repl_step(VM *vm)
         console_puts(SK_PROMPT_TEXT);
     }
 
-    sk_readline(input, sizeof(input));
+    sk_readline(input, sizeof(input), vm);
 
     if (input[0] == '\0') {
         console_puts(" ok\n");
@@ -397,7 +420,7 @@ void sk_repl_run(VM *vm)
             console_puts(SK_PROMPT_TEXT);
         }
 
-        sk_readline(input, sizeof(input));
+        sk_readline(input, sizeof(input), active);
 
         if (input[0] == '\0') {
             console_puts(" ok\n");