docs: catalog query pushdown design, marked proposed

Adds the design for executing a whole eligible read query at a catalog
provider -- optional read-query preparation and streaming, the Python provider
API, DuckDB 1.5 explicit execution and DuckDB 2.0 automatic pushdown -- and
registers it in the protocol index, the proposed-changes list and the nav.

Labelled **informative, proposed** in all three places on purpose. It is not
an extension of the filter-v2 contract and nothing implements it; filing it as
a design without that label would let a reader mistake it for a spec that
ports are behind on.

Author: rustyconover

PROTOCOL.md

@@ -6,6 +6,7 @@ The canonical protocol documents live in [`docs/protocol/`](docs/protocol/index.
 - [VGI Filter Encoding v2 — normative proposed specification](docs/protocol/vgi-filter-encoding-v2-spec.md)
 - [Runtime-filter artifacts — proposed optional extension](docs/protocol/vgi-runtime-filter-artifacts.md)
 - [DuckDB filter adapter — informative implementation guide](docs/protocol/vgi-duckdb-filter-adapter.md)
+- [Catalog query pushdown — proposed implementation design](docs/protocol/vgi-catalog-query-pushdown-design.md)
 - [DuckDB 2.0 protocol design/audit report — informative](docs/protocol/vgi-protocol-proposed-changes.md)
 
 The executable language-neutral conformance corpus and retargetable worker runner are in

docs/protocol/index.md

@@ -8,6 +8,7 @@ Each document declares its own status; “proposed” means the contract is stil
 | [VGI Filter Encoding v2](vgi-filter-encoding-v2-spec.md) | **Normative, proposed** | Engine-neutral wire format, semantics, validation, and conformance requirements for `vgi.filters.v2`. |
 | [VGI Runtime-Filter Artifacts](vgi-runtime-filter-artifacts.md) | **Normative optional extension, proposed** | Additional requirements for capability-gated advisory runtime-pruning artifacts. Base filter-v2 conformance does not require this extension. |
 | [VGI DuckDB Filter Adapter](vgi-duckdb-filter-adapter.md) | **Informative implementation guide, proposed** | DuckDB 1.5 and 2.0 mappings into the normative filter-v2 wire contract. |
+| [VGI catalog query pushdown](vgi-catalog-query-pushdown-design.md) | **Informative implementation design, proposed** | Optional read-query preparation and streaming, Python provider API, DuckDB 1.5 explicit execution, and DuckDB 2.0 automatic pushdown. |
 | [Proposed VGI protocol changes for DuckDB 2.0](vgi-protocol-proposed-changes.md) | **Informative design/audit report** | Rationale, migration inventory, implementation status, and deferred protocol decisions. It is not an implementation specification. |
 
 ## Reading order
@@ -16,6 +17,10 @@ Protocol implementers should begin with the normative [Filter Encoding v2](vgi-f
 runtime-artifact extension only when implementing one of its negotiated algorithms. Engine adapters should then use
 the relevant implementation guide without treating engine-private classes as wire types.
 
+The [catalog query pushdown design](vgi-catalog-query-pushdown-design.md) describes a separate optional
+capability for executing whole eligible read queries at a catalog provider. It is a proposal, not
+an implemented extension of the filter-v2 contract.
+
 The language-neutral filter corpus at `conformance/filter-v2/` contains structural JSON cases and executable Arrow
 IPC vectors. Its worker runner sends the portable cases through the public VGI client and can target any SDK's
 standard example worker.