Function wait_for_vram 

pub fn wait_for_vram(
    ledger: &mut VramLedger,
    budget_mb: usize,
    task: &str,
    config: &WaitConfig,
    profiler: &mut GpuProfiler,
) -> Result<u64, GpuError>

Poll the ledger until VRAM budget is available.

Returns Ok(()) when reservation is acquired, or Err(Timeout) when timeout is exceeded.