feat: MVCC, wire protocol, HTTP API, schema system, CLI shell — 39 tests

- MVCC: multi-version concurrency control with snapshot isolation
- Deadlock detection: wait-for graph with cycle detection
- Wire Protocol: binary message format, 16 types, big-endian serialization
- HTTP/REST API: router with CORS, path params, JSON
- Schema system: types, links, properties, migration diffing
- CLI: interactive query shell (bara shell)
- 18 new tests (39 total, all passing)

src/barabadb/fts/engine.nim

@@ -4,7 +4,6 @@ import std/strutils
 import std/unicode
 import std/math
 import std/algorithm
-import std/sets
 
 type
   TermFreq* = Table[string, int]