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>

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