feat: add ParquetLookupProvider and SqliteLookupProvider behind feature gates

Ports the two storage backends proven in the df-vector-search benchmark POC
into the library as optional, feature-gated providers.

- `parquet-provider` feature: ParquetLookupProvider — concurrent row-group
  reads from any ObjectStore (S3, local FS) with pre-cached parquet footers
  and optional RowSelection for page-skip optimisation
- `sqlite-provider` feature: SqliteLookupProvider — B-tree point lookups via
  a WAL-mode connection pool; builds from parquet on first run
- `keys` module (always compiled): pack_key / unpack_key / DatasetLayout —
  shared key encoding utilities extracted from indexing.rs
- Integration tests for both providers (9 tests total)

Breaking change in Cargo.toml: tokio gains the "sync" feature unconditionally
(needed by SqliteLookupProvider's Semaphore; tokio was already a hard dep).

Author: anoop-narang

Cargo.lock

@@ -5,14 +5,30 @@ edition = "2024"
 description = "DataFusion extension for USearch HNSW vector similarity search with adaptive WHERE clause filtering"
 license = "MIT OR Apache-2.0"
 
+[features]
+parquet-provider = ["dep:parquet", "dep:object_store", "dep:bytes"]
+sqlite-provider  = ["dep:rusqlite", "dep:serde_json", "dep:parquet"]
+
 [dependencies]
-tracing     = "0.1"
-datafusion  = "51.0.0"
-usearch     = "2.24.0"
-arrow-array = "57.2.0"
+tracing      = "0.1"
+datafusion   = "51.0.0"
+usearch      = "2.24.0"
+arrow-array  = "57.2.0"
 arrow-schema = "57.2.0"
-async-trait = "0.1"
-tokio       = { version = "1", features = ["rt-multi-thread", "macros"] }
+async-trait  = "0.1"
+futures      = "0.3"
+# "sync" adds tokio::sync::Semaphore, used by SqliteLookupProvider's connection pool
+tokio        = { version = "1", features = ["rt-multi-thread", "macros", "sync"] }
+
+# parquet-provider
+parquet      = { version = "57.2.0", optional = true, features = ["async", "object_store"] }
+object_store = { version = "0.12",   optional = true }
+bytes        = { version = "1",      optional = true }
+
+# sqlite-provider
+rusqlite     = { version = "0.32", optional = true, features = ["bundled"] }
+serde_json   = { version = "1",    optional = true }
 
 [dev-dependencies]
-tokio = { version = "1", features = ["rt-multi-thread", "macros"] }
+tokio    = { version = "1", features = ["rt-multi-thread", "macros"] }
+tempfile = "3"