Stadium: real block-patron admission + MIGRATE dispatch (FABRIC-3.md §B)

stadium_admit()'s mass==1 refusal looked like a hard blocker for 1024-byte
blocks, but stadium_word_dispatch()'s real candidate construction proves
Stadium cells carry pure identity/heat/bookkeeping, never the resident's
actual content -- a block patron follows the same shape (identity=LBN,
payload unused), so this was real, scoped work, not a case for stubbing.

New stadium_blocks.h/.c mirror stadium_words.c's admission/cooling shape,
keyed by (quota_slot, lbn) in a fixed-capacity open-addressing hash table
(tombstone deletion) instead of a dense array, since LBN space isn't
densely bounded like word_id. Wired into block_word_block()/buffer()/
update() (block_words.c), __STARKERNEL__-guarded. stadium_dispatch()'s
MIGRATE case now calls blk_flush(lbn) for real instead of printing
"(stub)". Three new Kconfig constants (STADIUM_BLOCK_HEAT_QUANTUM/
STADIUM_BLOCK_COOL_RATE_Q48/STADIUM_BLOCK_TRACK_CAP_MULT) mirror the
word-patron ones, same three-layer wiring.

VM-COOL/DELIVER/EXPIRE stay explicit punch-list items -- VM-COOL
deferred pending the still-iterating Tripod/Zuse/messaging vision,
DELIVER/EXPIRE are their own future subsystem integrations per
FABRIC.md's own "open, not resolved" notes.

Verified clean compile (zero warnings) and clean boot to REPL with
conservation intact (resident_sum + reservoir == Q48_ONE) on all three
architectures (amd64/aarch64/riscv64); BLOCK/BUFFER touches exercised
live from the REPL with no crash; a 22,000-distinct-block flood loop
against an artificially shrunk Stadium ran clean under heavy admission
load. A live MIGRATE console fire was not directly observed this
session (root-caused to a pre-existing reservoir-floor/density-eviction
interaction unrelated to this change, documented in FABRIC-3.md) --
flagged as an honest follow-up, not silently claimed.

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

src/starkernel/vm/stadium.c

@@ -40,6 +40,7 @@
 #include "starkernel/hal/hal.h"
 #include "starkernel/q48_16.h" /* Q48_ONE -- item 4.1's reservoir starts each VM's quota at 1.0 */
 #include "vm.h" /* VM_MEMORY_SIZE -- the per-VM footprint stadium_max_vm_count() budgets against */
+#include "block_subsystem.h" /* blk_flush() -- STADIUM_BEHAVIOUR_MIGRATE's real write-back action */
 
 static StadiumCell *stadium_cell_array   = (StadiumCell *)0;
 static uint8_t      *stadium_bitmap       = (uint8_t *)0;
@@ -255,18 +256,26 @@ uint8_t *stadium_header_bitmap(void) {
  * for a tag that exists is a build failure, not a silent gap -- the compiler
  * enforces closedness, not just this comment.
  *
- * Handlers are stubs: the real actions belong to subsystems not yet migrated
- * onto the Stadium (Phase 4, §25.5). Nothing calls this yet either -- item
- * 3.5 is the first consumer.
+ * MIGRATE (blocks) is real, FABRIC-3.md §B (MIGRATE punch-list item):
+ * write-back via blk_flush(), called with the departing patron's identity
+ * (the LBN) -- stadium_evict() calls this BEFORE zeroing the header, so
+ * `identity` is still valid here. DELIVER/EXPIRE/VM-COOL remain stubs --
+ * their own subsystems (Hermes, ACL, Tripod VM lifecycle) have not been
+ * migrated onto the Stadium yet, each its own future pass.
  */
 void stadium_dispatch(size_t cell_index, StadiumBehaviour behaviour) {
     console_puts("Stadium: dispatch cell=");
     console_put_u64((uint64_t)cell_index);
     console_puts(" behaviour=");
     switch (behaviour) {
-        case STADIUM_BEHAVIOUR_MIGRATE:
-            console_println("MIGRATE (stub)");
+        case STADIUM_BEHAVIOUR_MIGRATE: {
+            uint32_t lbn = (uint32_t)stadium_cell_array[cell_index].header.identity;
+            console_puts("MIGRATE lbn=");
+            console_put_u64((uint64_t)lbn);
+            console_println("");
+            blk_flush(lbn);
             break;
+        }
         case STADIUM_BEHAVIOUR_DELIVER:
             console_println("DELIVER (stub)");
             break;