Buffer Pool Manager & LRU Eviction

The in-memory buffer pool caches 4 KB disk pages in memory frames. It tracks pin counts and dirty page flags to manage Least Recently Used (LRU) eviction.

// Frame metadata tracking in buffer pool
pub struct FrameHeader {
    pub page_id: PageId,
    pub pin_count: u32,
    pub is_dirty: bool,
    pub last_accessed_lsn: u64,
}