Add TCP server with EDN protocol

[verberktstan]

Implements the TCP layer and EDN wire protocol, making Bitten accessible as a running server.

• src/server.clj - ServerSocket accept loop, one thread per connection, handle-request dispatch
• src/protocol.clj - EDN parse/serialize; malformed input returns {:error …}
• :transact operation passes records directly to db/upsert!
• bb start task with optional PORT and DB_PATH env vars

Some example EDN payloads:

{:op :transact :records [{:db/entity "user/1" :user/name "Alice"}]}
{:op :query :e "user/1" :as-of-valid "2024-06-01"}
{:op :ping}