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/word_source/block_words.c

@@ -270,8 +270,17 @@ static vaddr_t blk_vm_assign(VM *vm, uint32_t lbn) {
  * the epoch first (Milestone 2h, see blk_vm_check_epoch()) since this
  * function walks vm->blk_vm_lbn[]/vm->blk_vm_cbuf[] directly rather than
  * through blk_vm_find() -- a stale-epoch dirty slot must be discarded, not
- * flushed onto whatever device now owns that LBN. */
-static void blk_vm_flush_all(VM *vm) {
+ * flushed onto whatever device now owns that LBN.
+ *
+ * Non-static: this is the entire implementation behind SAVE-BUFFERS
+ * (block_word_save_buffers() below is a one-line wrapper) -- exposed
+ * (declared in block_words.h) so kernel-side code can reuse the exact
+ * same flush path outside the word-dispatch mechanism, e.g. sk_repl_idle()
+ * (see FABRIC.md/FABRIC-2.md Section V item 6). Cheap to call when
+ * nothing is dirty: every check below is a small fixed-size scan
+ * (BLK_VM_SLOTS here, DISK_CACHE_SLOTS per device inside blk_flush()),
+ * no I/O happens unless something actually needs writing. */
+void blk_vm_flush_all(VM *vm) {
     blk_vm_check_epoch(vm);
     for (int i = 0; i < BLK_VM_SLOTS; i++) {
         if (vm->blk_vm_cbuf[i] != NULL && vm->blk_vm_dirty[i]) {

src/word_source/include/block_words.h

@@ -137,6 +137,21 @@ void mark_buffer_dirty(VM * vm);
  */
 void save_all_buffers(VM * vm);
 
+/**
+ * @brief Sync every dirty VM block-window slot to the C-layer block cache,
+ *        then flush the whole block subsystem to physical media --
+ *        the complete SAVE-BUFFERS implementation (unlike
+ *        save_all_buffers() above, which only calls blk_flush() and so
+ *        misses any BUFFER-modified-but-not-yet-UPDATE'd content still
+ *        sitting only in vm->memory's window). Cheap to call when nothing
+ *        is dirty -- every check inside is a small fixed-size scan, no
+ *        I/O happens unless something actually needs writing. Safe to
+ *        call from outside word dispatch (e.g. a periodic idle-tick
+ *        auto-flush); vm must be non-NULL.
+ * @param vm Pointer to the Forth virtual machine instance
+ */
+void blk_vm_flush_all(VM * vm);
+
 /*
  * @brief Marks all buffers as empty
  * @param vm Pointer to the Forth virtual machine instance