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>

src/starkernel/vm/stadium.c

@@ -150,4 +150,23 @@ void stadium_dispatch(size_t cell_index, StadiumBehaviour behaviour) {
     }
 }
 
+/*
+ * FABRIC.md §19.2/§19.3: density is heat / mass, read on demand from fields
+ * already in the header -- not a value a scheduler maintains. mass == 0
+ * (an empty or never-admitted slot; everything is zero-initialized until
+ * something is actually born into the Stadium, which nothing yet does)
+ * returns 0 rather than dividing by zero. An out-of-range cell_index also
+ * returns 0 -- there is no patron there to be dense.
+ */
+uint64_t stadium_density(size_t cell_index) {
+    StadiumPatronHeader *header;
+
+    if (cell_index >= stadium_ncells) return 0;
+
+    header = &stadium_cell_array[cell_index].header;
+    if (header->mass == 0) return 0;
+
+    return header->heat / (uint64_t)header->mass;
+}
+
 #endif /* __STARKERNEL__ */