refactor(trie): replace bonsai-trie with pathfinder-inspired Merkle trie

[kariy]

Summary

• Replace bonsai-trie dependency with a new Merkle Patricia Trie implementation ported from pathfinder, eliminating the external trie dependency entirely
• Simplify DB schema from 9 trie tables down to 5, with leaf hashes embedded directly in parent nodes (LeafBinary/LeafEdge variants) instead of separate leaf tables
• Decouple trie computation from DB with an in-memory-first approach: load existing trie into MemStorage, compute all mutations in memory, then persist the resulting TrieUpdate to DB
• Add LRU cache to DbTrieStorage for read-only operations (proof generation, root queries)
• Add comprehensive documentation in docs/trie.md covering architecture, node types, persistence flow, and design decisions

Key changes by commit

1. d59705af — Core rewrite: New MerkleTree<H, HEIGHT> with Storage trait, MemStorage, node types, proof generation. Remove bonsai-trie, slab deps
2. f396e5e8 — LRU cache: Add quick_cache-backed LRU to DbTrieStorage (4096 entries) for read-heavy proof/root operations
3. bafc21e7 — Embed leaf hashes: Remove 3 leaf tables, store leaf hashes in LeafBinary/LeafEdge parent nodes for correct historical access
4. ffa72a3b — In-memory-first mutation: MemStorage uses HashMap for arbitrary indices, add load_trie_to_memory() BFS loader, TrieWriter now loads→computes→persists
5. e5fb14b6 — Documentation: Add docs/trie.md, update docs/database.md with trie tables

Architecture

Provider (TrieWriter)
  │  load → compute → persist
  ├──────────────────────┐
  │                      │
katana-trie          katana-db
  MerkleTree           TrieDbFactory
  Storage trait        DbTrieStorage (read, cached)
  MemStorage           MemStorage (write, in-memory)
  ProofNode            load_trie_to_memory
                       persist_trie_update

Test plan

• cargo nextest run -p katana-trie — core trie unit tests
• cargo nextest run -p katana-db — DB storage, loading, multi-block tests
• cargo nextest run -p katana-provider -E 'not test(fork)' — provider integration
• cargo nextest run -p katana-stage — sync stage tests
• ./scripts/clippy.sh — lint clean
• cargo +nightly-2025-02-20 fmt --all — format clean

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