kmb-store: Page-based B+tree projection store with MVCC

This crate provides a persistent key-value store optimized for projection state in Kimberlite. Key features:

• B+tree indexing: Efficient point lookups and range scans
• MVCC: Multi-Version Concurrency Control for point-in-time queries
• Page-based storage: 4KB pages with CRC32 integrity checks
• Crash recovery: Superblock with applied position tracking

Architecture

┌─────────────────────────────────────────────────────┐
│  ProjectionStore (public API)                       │
├─────────────────────────────────────────────────────┤
│  BTree (search, insert, delete, scan with MVCC)     │
├─────────────────────────────────────────────────────┤
│  PageCache (LRU cache with page-aligned I/O)        │
├─────────────────────────────────────────────────────┤
│  Page Layer (4KB pages, CRC32, slot directory)      │
└─────────────────────────────────────────────────────┘

Usage

use kimberlite_store::{ProjectionStore, BTreeStore, WriteBatch, WriteOp};
use kimberlite_types::Offset;

// Open or create a store
let mut store = BTreeStore::open("data/projections")?;

// Apply a batch of changes
let batch = WriteBatch::new(Offset::new(1))
    .put(table_id, key, value)
    .delete(table_id, other_key);
store.apply(batch)?;

// Point-in-time query
let value = store.get_at(table_id, &key, Offset::new(1))?;