starkernel: item 3.3 -- behaviour enumeration and dispatch

Punch list §25 item 3.3 complete.

StadiumBehaviour (stadium.h) enumerates exactly the four tags §18.3
already names -- MIGRATE, DELIVER, EXPIRE, COOL -- mapped from §17.1's
patron table: blocks->MIGRATE, messages->DELIVER, ACLs->EXPIRE, words
and VMs both->COOL. Nothing invented; the tag set and mapping were
already in the document.

stadium_dispatch(cell_index, behaviour) dispatches on the tag only,
never asks what kind of patron departed. Handlers are stubs -- the
real actions belong to subsystems not yet migrated onto the Stadium
(Phase 4). Nothing calls stadium_dispatch() yet; item 3.5 is its first
consumer.

The switch is exhaustive with no default case, making §13's "closed
enumeration, fixed at build time" a compiler-enforced property under
this project's -Wall -Werror rather than just prose. Verified live:
temporarily deleted the COOL case, rebuild failed with
error: enumeration value 'STADIUM_BEHAVIOUR_COOL' not handled in
switch [-Werror=switch], restored it, confirmed clean again.

The header's behaviour field stays uint8_t, not the enum type itself,
since C does not guarantee an enum's underlying type and that field's
offset is load-bearing for item 3.1's validated 64-byte layout.

Verified: three-architecture boot (amd64, aarch64, riscv64), all
reaching ok> with identical dict_hash=0x3d4e1daf289da94f matching the
item-3.2 baseline.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

include/starkernel/vm/stadium.h

@@ -63,7 +63,11 @@ typedef struct {
                              * enforcement of that bound is item 3.5's scope, not this one's. */
     uint16_t mass;         /* offset 28 -- cells this patron occupies (§19.2) */
     uint8_t  flags;        /* offset 30 -- bit 0 = pin; remaining bits reserved */
-    uint8_t  behaviour;    /* offset 31 -- code field, closed enumeration (§18.3) */
+    uint8_t  behaviour;    /* offset 31 -- code field. Valid values are StadiumBehaviour (§18.3)
+                             * tags cast to uint8_t -- kept as uint8_t rather than the enum type
+                             * itself since C does not guarantee an enum's underlying type, and
+                             * this field's offset is load-bearing for the 64-byte layout item
+                             * 3.1 validated. */
     uint8_t  payload[32];  /* offset 32 -- inline payload, used when mass == 1 */
 } StadiumPatronHeader;
 
@@ -138,6 +142,42 @@ StadiumCell *stadium_cells(void);
  */
 uint8_t *stadium_header_bitmap(void);
 
+/*
+ * StadiumBehaviour - the closed code-field enumeration (FABRIC.md §13, §18.3).
+ * The engine dispatches on this tag and never asks a patron what kind it is
+ * -- §3's entire point. Two patrons may share a tag: a VM's tag is COOL, the
+ * same tag a word carries (§18.3). Mapped from §17.1's patron table:
+ *
+ *   MIGRATE -- blocks: reap event is migration back to Artemis (§17.2)
+ *   DELIVER -- messages: reap event is delivery
+ *   EXPIRE  -- ACLs: reap event is TTL expiry
+ *   COOL    -- words and VMs: reap event is cooling off the floor
+ *
+ * Closed and fixed at build time -- see stadium_dispatch()'s exhaustive
+ * switch for how the compiler enforces that.
+ */
+typedef enum {
+    STADIUM_BEHAVIOUR_MIGRATE = 0,
+    STADIUM_BEHAVIOUR_DELIVER,
+    STADIUM_BEHAVIOUR_EXPIRE,
+    STADIUM_BEHAVIOUR_COOL
+} StadiumBehaviour;
+
+/*
+ * stadium_dispatch - Calls the behaviour handler for a patron's code field.
+ * The engine calls this at reap and never asks what kind of patron departed
+ * (§3, §18.3) -- only cell_index and behaviour cross this boundary.
+ *
+ * Handlers are stubs today: the real migrate-to-Artemis / deliver / expire /
+ * cool actions belong to their own subsystems, which have not been migrated
+ * onto the Stadium yet (Phase 4, §25.5). Nothing calls stadium_dispatch()
+ * yet either -- that begins with item 3.5 (admission and eviction).
+ *
+ * @param cell_index  Index into the Stadium of the patron header dispatching.
+ * @param behaviour   Which of the closed tag set to invoke.
+ */
+void stadium_dispatch(size_t cell_index, StadiumBehaviour behaviour);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_STADIUM_H */