refactor(v3): the block subsystem's state is a chain there can be two of; blk_subsys_init takes no VM

The global state becomes struct blk_chain, reached through a current
pointer: blk_chain_default, blk_chain_new, blk_chain_select.  Nothing
that uses the one chain changes.  blk_subsys_init loses its VM argument,
which was stored and never used.  docs/v4.0.0/MESH.md 8.4.

Accepted on the v3 configuration: amd64, aarch64 and riscv64 reach the
zuse prompt, no UNKNOWN WORD, PARITY:M7.1a hash 0x08873e0f44b7cb2a on
all three, as on 2026-10-03.  logs/20261006-202918, -203036, -203230.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

kernel/src/capsule/capsule_loader.c

@@ -95,7 +95,9 @@
 
 int capsule_blk_init(void *vm, uint8_t *ram_buf, size_t ram_size, uint8_t *krd_buf)
 {
-    int rc = blk_subsys_init((VM *)vm, ram_buf, ram_size);
+    int rc;
+    (void)vm;   /* the block subsystem no longer takes one */
+    rc = blk_subsys_init(ram_buf, ram_size);
     if (rc != 0) return rc;
     return blk_subsys_add_raw_device(krd_buf, KRD_MAX_BLOCKS);
 }

kernel/src/vm/bootstrap/sk_vm_bootstrap.c

@@ -303,7 +303,7 @@ int sk_vm_bootstrap_parity(ParityPacket *out) {
 
     /* Initialize block subsystem for POST — mirrors hosted main.c init order.
      * Without this, all block word tests fail with vm->error=1 (blk_is_valid→0). */
-    blk_subsys_init(vm, sk_post_blk_ram, sizeof(sk_post_blk_ram));
+    blk_subsys_init(sk_post_blk_ram, sizeof(sk_post_blk_ram));
 
     /* Run full POST (Power-On Self Test) */
     console_println("POST: Running comprehensive test suite...");

v3/include/block_subsystem.h

@@ -327,7 +327,18 @@ typedef struct {
 enum { BLK_FMT_FORMATTED = 0, BLK_FMT_PROVISIONAL = 1 };
 
 /* ===== Public API ===== */
-int blk_subsys_init(VM *vm, uint8_t *ram_base, size_t ram_size);
+int blk_subsys_init(uint8_t *ram_base, size_t ram_size);
+
+/* A chain is fast RAM and the devices chained after it.  blk_chain_default
+ * is the one there has always been.  blk_chain_new makes another, not yet
+ * initialized, or returns 0 if there is no memory.  blk_chain_select makes
+ * every blk_* call from now on act on `c`, blk_subsys_init included, and
+ * returns the chain that was selected before; a null `c` selects nothing
+ * new.  docs/v4.0.0/MESH.md 8.4. */
+typedef struct blk_chain blk_chain_t;
+blk_chain_t *blk_chain_default(void);
+blk_chain_t *blk_chain_new(void);
+blk_chain_t *blk_chain_select(blk_chain_t *c);
 
 int blk_subsys_attach_device(struct blkio_dev *dev);
 

v3/src/block_subsystem.c

@@ -167,8 +167,11 @@ struct blk_dev_slot {
 };
 
 /* ===== global state ===== */
-static struct {
-    VM       *vm;
+/* A chain: fast RAM and the devices chained after it.  There is the one
+ * v3 has always had (blk_chain_default) and there can be others
+ * (blk_chain_new); every blk_* call acts on the one selected
+ * (blk_chain_select).  docs/v4.0.0/MESH.md 8.4. */
+struct blk_chain {
     uint8_t  *ram_base;
     size_t    ram_size;
     uint8_t   dirty_ram[BLK_RAM_BLOCKS];
@@ -200,7 +203,20 @@ static struct {
     uint32_t          reloc_count;
 
     int initialized;
-} g = {0};
+};
+static struct blk_chain  g_default;
+static struct blk_chain *g_current = &g_default;
+#define g (*g_current)
+
+blk_chain_t *blk_chain_default(void) { return &g_default; }
+
+blk_chain_t *blk_chain_new(void) { return (blk_chain_t *) calloc(1, sizeof(struct blk_chain)); }
+
+blk_chain_t *blk_chain_select(blk_chain_t *c) {
+    blk_chain_t *was = g_current;
+    if (c) g_current = c;
+    return was;
+}
 
 /* Redirect lbn through the relocation table if it's been moved elsewhere.
  * The single choke point every public LBN-consuming entry point below
@@ -644,11 +660,10 @@ static inline uint64_t blk_get_timestamp(void) {
 
 /* ===== public API ===== */
 
-int blk_subsys_init(VM *vm, uint8_t *ram_base, size_t ram_size) {
-    if (!vm || !ram_base || ram_size < ((size_t) BLK_RAM_BLOCKS * BLK_FORTH_SIZE))
+int blk_subsys_init(uint8_t *ram_base, size_t ram_size) {
+    if (!ram_base || ram_size < ((size_t) BLK_RAM_BLOCKS * BLK_FORTH_SIZE))
         return BLK_EINVAL;
     memset(&g, 0, sizeof(g));
-    g.vm          = vm;
     g.ram_base    = ram_base;
     g.ram_size    = ram_size;
     g.ram_user    = (BLK_RAM_BLOCKS > BLK_FORTH_SYS_RESERVED)

v3/src/main.c

@@ -316,9 +316,9 @@ static int init_vm_and_subsystem(VM* vm)
     }
 
     /* Initialize block subsystem */
-    extern int blk_subsys_init(VM* vm, uint8_t* ram_base, size_t ram_size);
+    extern int blk_subsys_init(uint8_t* ram_base, size_t ram_size);
     static uint8_t blk_ram[BLK_RAM_BLOCKS * BLK_FORTH_SIZE];
-    int rc = blk_subsys_init(vm, blk_ram, sizeof(blk_ram));
+    int rc = blk_subsys_init(blk_ram, sizeof(blk_ram));
     if (rc != 0)
     {
         fprintf(stderr, "Failed to initialize block subsystem (rc=%d)\n", rc);