Implement the on-disk open-addressing hash table for storing trie pages:
- htfile.go: HT file layout with header, meta pages, and data pages
- metamap.go: in-memory meta byte map with dirty page tracking
- probe.go: triangular probing with xxhash64 page ID hashing
- db.go: Bitbox DB with StorePage, LoadPage, DeletePage, FlushMeta, Sync
The hash table uses 1-byte meta tags (top 7 bits of hash) for fast
filtering before reading full 4096-byte data pages. Triangular probing
with power-of-2 capacity guarantees all buckets are visited.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
