refactor(v3): the block subsystem is one chain again

blk_chain_default, blk_chain_new and blk_chain_select (0e761cb1) were for
a node's own drive, which was withdrawn on 2026-10-07 (docs/v4.0.0/MESH.md
8.5).  They are taken out.  What remains of that change is that
blk_subsys_init takes no VM.

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 before.  logs/20261007-082647, -082752, -082938.

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

v3/src/block_subsystem.c

@@ -167,11 +167,7 @@ struct blk_dev_slot {
 };
 
 /* ===== global state ===== */
-/* 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 {
+static struct {
     uint8_t  *ram_base;
     size_t    ram_size;
     uint8_t   dirty_ram[BLK_RAM_BLOCKS];
@@ -203,20 +199,7 @@ struct blk_chain {
     uint32_t          reloc_count;
 
     int initialized;
-};
-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;
-}
+} g = {0};
 
 /* Redirect lbn through the relocation table if it's been moved elsewhere.
  * The single choke point every public LBN-consuming entry point below