Function configureGpuByteBudget

Configure the budget once at startup from the resolved config value (or the ?gpuBudgetMB URL param, which the caller passes in):

• null / undefined / NaN → auto-size from available ambient memory signals and any explicit cache-pool override. NaN also warns.
• 0 → disable byte-budget eviction (unbounded resident geometry).
• a positive number → pin the budget to exactly that many bytes.