docs: add project readme

README.md

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+# cobalt
+
+An embedded, log-structured merge-tree (LSM) key/value store written in Zig.
+
+Cobalt keeps recent writes in an in-memory skiplist backed by a write-ahead
+log, flushes them to immutable SSTables, and reclaims space through leveled
+compaction. Reads merge the memtable and every SSTable, honouring MVCC
+sequence numbers so snapshots and transactions observe a consistent view.
+
+## Features
+
+- Write-ahead log with single and batched frames, CRC framing, and
+  torn-tail recovery.
+- Skiplist memtable with forward and reverse ordered cursors.
+- SSTable format with a block index, per-table bloom filter, RLE block
+  compression, and CRC-checked blocks.
+- LRU block cache shared across readers.
+- MVCC snapshots and optimistic transactions with read/write conflict
+  detection.
+- Leveled compaction that keeps levels disjoint and drops shadowed
+  tombstones at the bottom level.
+- Forward, reverse, range, and prefix scans that merge all sources.
+- Atomic write batches and a manifest describing the level of each table.
+
+## Layout
+
+| File | Responsibility |
+|------|----------------|
+| `src/encode.zig` | byte encoding helpers, record and key types |
+| `src/memtable.zig` | skiplist memtable and cursors |
+| `src/wal.zig` | write-ahead log and replay |
+| `src/sstable.zig` | SSTable builder, reader, and cursors |
+| `src/bloom.zig` | bloom filter |
+| `src/cache.zig` | LRU block cache |
+| `src/compress.zig` | RLE block compression |
+| `src/batch.zig` | write batches |
+| `src/stats.zig` | database statistics |
+| `src/txn.zig` | MVCC transactions |
+| `src/db.zig` | store facade, compaction, and iterators |
+| `src/main.zig` | command-line interface |
+| `src/bench.zig` | throughput benchmark |
+
+## Build
+
+```sh
+zig build            # build the library, cli, and benchmark
+zig build test       # run the unit tests
+zig build run -- <dir> <command> [args]
+zig build bench -- [operations]
+```
+
+## CLI
+
+```sh
+cobalt <dir> put <key> <value>
+cobalt <dir> get <key>
+cobalt <dir> del <key>
+cobalt <dir> scan [start] [end]
+cobalt <dir> rscan [start] [end]
+cobalt <dir> prefix <prefix>
+cobalt <dir> compact
+cobalt <dir> stats
+cobalt <dir> verify <n>
+```