starkernel: item 3.2 -- Stadium boot-time allocation

Punch list §25 item 3.2 complete.

stadium_boot_init() (src/starkernel/vm/stadium.c) sizes the global
cell array at boot from a real memory-budget query rather than a
hardcoded count: pmm_get_stats().free_bytes at the point of
allocation, times the new STADIUM_MEMORY_PERCENT Kconfig symbol
(default 1%), rounded down to whole 64-byte cells. Matches §17.6's
position (b) literally. Also allocates the header/continuation
discriminator bitmap item 3.1 declared but did not allocate. Both are
kmalloc'd and explicitly zero-filled (kmalloc does not zero).

Called from kernel_main.c immediately before sk_vm_bootstrap_parity(),
i.e. before any VM exists (§6). Failure is soft -- logs and continues,
does not halt boot -- matching the existing precedent one line below
it (VM bootstrap parity failure does the same).

Added a "Stadium: N cells (M KB)" boot console line at the allocation
site so the acceptance logs are evidence the array was actually
allocated, not just that the kernel still boots -- the same blind spot
item 3.1's uncompiled-header gap exposed.

Verified: three-architecture boot (amd64, aarch64, riscv64), all
reaching ok> with identical dict_hash=0x3d4e1daf289da94f matching the
item-3.1 baseline, and the Stadium boot line confirmed present in all
three serial logs (amd64: 74234 cells/4639 KB, aarch64: 161329
cells/10083 KB, riscv64: 76122 cells/4757 KB).

Not built here, reported per §25.0 rule 3: per-VM free lists (§22.3)
-- granted when Hera assigns quota, not this item's scope.

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

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