stadium: make VM population bound RAM-derived, not a static array of 4

Replaces STADIUM_MAX_VM_COUNT (Kconfig, hardcoded default 4) with a
boot-time computation, mirroring the pattern stadium_boot_init() already
used for the cell pool. New Kconfig STADIUM_VM_MEMORY_PERCENT (default
50): max_vm_count = (kmalloc_get_stats().free_bytes after the cell array
* STADIUM_VM_MEMORY_PERCENT / 100) / VM_MEMORY_SIZE, floored to 1, no
ceiling (population is not knowable in advance - could be 4, could be
4000). stadium_quotas and word_slots (plus stat_promotions/stat_evictions)
are now kmalloc'd to the computed count instead of declared with a macro.
New accessor stadium_max_vm_count() replaces every STADIUM_MAX_VM_COUNT
reference, including capsule_birth.c's birth-refusal gate.

Two things found and fixed along the way:

- The existing cell-pool budget was sourced from pmm_get_stats(), which
  reflects physical pages PMM hasn't handed to any subsystem yet - but
  the actual allocation is kmalloc(), which draws from the separate,
  fixed-size heap kmalloc_init() (M6) already carved out of PMM before
  stadium_boot_init() ever runs. Budgeting against PMM's leftover and
  allocating from the kmalloc heap are two different pools. Both the
  cell budget and the new VM-count budget now source from
  kmalloc_get_stats() instead.

- stadium_owner[] (which VM's quota owns each cell) was uint8_t, capped
  at 255 slots by a compile-time assert tied to the old macro. Widened
  to uint16_t (65535 slots of headroom) with a runtime clamp + log if
  the computed count ever exceeds that, since there's no ceiling anymore.

Three-arch QEMU acceptance: all clean to ok>, computed VM count genuinely
differs by actual available RAM (amd64/riscv64: 50 slots at -m 1024,
aarch64: 101 slots), Stadium conservation invariant identical across all
three (resident_sum=43691 reservoir=21845 sum=65536).
logs/20260815-080526/amd64, logs/20260815-080826/aarch64,
logs/20260815-080952/riscv64.

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

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