feat: add generic search client traits and adapters

Introduce a family of search client traits with blanket implementations
and adapter utilities to reduce boilerplate across backends.

New traits
----------
- ItemsClient: single-page item search (search required); defaults: item, items
- StreamItemsClient: streaming items across pages (search_stream required);
  defaults: collect_items, item_count, items_stream
- CollectionsClient: fetch all collections (collections required);
  default: collection point-lookup
- PagedCollectionsClient: cursor-paginated collections (collections_page required)
  for future backends that support paginated /collections
- StreamCollectionsClient: streaming collections (collections_stream required);
  default: collect_collections
- ArrowItemsClient (geoarrow feature): Arrow record batch output
  (search_to_arrow required); default: items_to_arrow
- TransactionClient: write operations

Blanket implementations
-----------------------
- CollectionsClient + Clone + Sync -> StreamCollectionsClient (eager fetch)
- ArrowItemsClient + Sync -> ItemsClient + StreamItemsClient (geoarrow feature)

Adapter utilities
-----------------
- PagedItemsStream<T>: wraps ItemsClient to provide StreamItemsClient via
  token/skip pagination using ItemCollection::next
- stream_pages_generic: free function driving the pagination loop
- stream_pages_collections_generic: collections equivalent for
  PagedCollectionsClient backends
- RecordBatchReaderAdapter<I> (geoarrow feature): bridges any
  Iterator<Item = Result<RecordBatch, E>> to arrow_array::RecordBatchReader

Documentation
-------------
Add docs/search-clients.md covering the trait family, blanket impls, adapter
conversion chart, pagination mechanics, and performance notes.

Author: bitner

.gitignore

@@ -9,3 +9,4 @@ node_modules/
 package-lock.json
 crates/wasm/tests/__screenshots__
 .yarn/
+planfor*.md

crates/core/Cargo.toml

@@ -28,13 +28,16 @@ geoarrow = [
 geoparquet = ["geoarrow", "dep:geoparquet", "dep:parquet"]
 
 [dependencies]
+async-stream.workspace = true
+futures.workspace = true
 arrow-array = { workspace = true, optional = true, features = ["chrono-tz"] }
 arrow-cast = { workspace = true, optional = true }
 arrow-json = { workspace = true, optional = true }
 arrow-schema = { workspace = true, optional = true }
 bytes.workspace = true
 chrono = { workspace = true, features = ["serde"] }
 cql2.workspace = true
+futures-core.workspace = true
 geo = { workspace = true, optional = true }
 geo-traits = { workspace = true, optional = true }
 geo-types = { workspace = true, optional = true }