starkernel: item 3.7 -- per-VM free lists (Phase 3 core complete, for real)

Punch list §25 item 3.7 complete. Added to §25.4 after starting item
4.1 surfaced it as an unbuilt prerequisite -- 3.6's earlier "Phase 3
core complete" claim is corrected in this same commit.

StadiumVMQuota table (size STADIUM_MAX_VM_COUNT, linearly searched by
vm_id -- capsule_birth.c's vm_id is monotonic and never reused, so it
cannot index a table directly, and a 4-entry scan costs nothing). New
per-cell stadium_owner byte array records which quota a cell belongs
to, needed so eviction returns a freed cell to the correct VM's list
and so eviction search stays scoped to the evicting VM's own residents
(quota isolation).

Free-list linkage reuses each cell's `link` field as a next-free
pointer while unresident -- link is documented only as generic "index
into the Stadium, not a pointer," so this is a repurposing, not a
header change. Does not answer the separate, still-open question of
which field carries a multi-cell patron's first continuation-cell
index; item 3.5's mass != 1 refusal stands exactly as it was.

Boot-time: every cell chained into one list in ascending index order,
granted whole to vm_id 0 (Hera), the only VM that exists. Ascending
order preserves item 3.6's "Hera is patron zero" invariant once real
birth-wiring lands.

stadium_admit()'s signature changed to take vm_id -- a change to code
shipped in item 3.5, amended there. Pops the calling VM's free-list
head first (O(1)); only falls back to a same-VM-scoped eviction search
if empty.

Caught a real bug before the boot run: the header zero-fill on
eviction (and the initial free-list build) both left contains == 0,
but 0 is Hera's valid index -- the same collision item 3.1's
STADIUM_CONTAINS_NONE fix addressed, recurring at a new site. Fixed by
explicitly setting contains = STADIUM_CONTAINS_NONE at both free-list
sites.

Explicitly out of scope, reported not invented: granting quota to any
VM other than Hera is capacity arbitration (item 1.3 left "how much
moves per transfer" open). stadium_owner is set once at boot and never
rewritten, so quota_slot_for_vm() refuses every vm_id != 0 permanently
until item 4.2 adds the grant path and owner-array writes.

Verified: three-architecture boot (amd64, aarch64, riscv64), all
reaching ok> with identical dict_hash=0x3d4e1daf289da94f matching the
item-3.6 baseline.

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

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