starkernel: item 3.4 -- density ranking

Punch list §25 item 3.4 complete.

stadium_density(cell_index) reads a header's heat and mass and returns
heat / mass -- a division on demand from fields already stored in the
cell, matching §19.3's "read, not computed by a scheduler" literally.
Stays valid Q48.16 without a special fixed-point routine, since heat
is already Q48.16 and mass is a plain integer divisor.

mass == 0 and an out-of-range cell_index both return 0 rather than
dividing by zero -- an empty or never-admitted slot has no footprint
to be dense within.

Deliberately not built here, per the item's own wording: finding the
densest or least-dense resident (§19.3's admission/eviction
comparison) is item 3.5's scope, not this one's. Nothing calls
stadium_density() yet either.

Verified: three-architecture boot (amd64, aarch64, riscv64), all
reaching ok> with identical dict_hash=0x3d4e1daf289da94f matching the
item-3.3 baseline.

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

include/starkernel/vm/stadium.h

@@ -178,6 +178,29 @@ typedef enum {
  */
 void stadium_dispatch(size_t cell_index, StadiumBehaviour behaviour);
 
+/*
+ * stadium_density - Heat / mass for the patron header at cell_index (FABRIC.md
+ * §19.2, §19.3). Read, not computed by a scheduler: both operands already
+ * live in the header, so this is a division on demand, not maintained
+ * bookkeeping. Result stays valid Q48.16, since heat is already Q48.16 and
+ * mass is a plain integer divisor.
+ *
+ * Returns 0 if mass is 0 -- an empty or never-admitted slot (everything is
+ * zero-initialized by stadium_boot_init() until something is actually born
+ * into the Stadium, which nothing yet does) has no footprint to be dense
+ * within, rather than a division by zero.
+ *
+ * Does not validate that cell_index actually holds a header rather than a
+ * continuation cell or an out-of-range index -- callers are expected to
+ * consult the item-3.1 discriminator bitmap first. Ranking (finding the
+ * densest or least-dense resident) is item 3.5's scope, not this one's;
+ * this function only supplies the per-cell value that comparison reads.
+ *
+ * @param cell_index  Index into the Stadium of the patron header to measure.
+ * @return Density in Q48.16, or 0 if the header's mass is 0.
+ */
+uint64_t stadium_density(size_t cell_index);
+
 #endif /* __STARKERNEL__ */
 
 #endif /* STARKERNEL_VM_STADIUM_H */