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...");