docs: add standalone KV indexer documentation (#6511)

Signed-off-by: PeaBrane <yanrpei@gmail.com>

docs/pages/components/router/standalone-indexer.md

+---
+# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+title: Standalone KV Indexer
+subtitle: Run the KV cache indexer as an independent service for querying block state
+---
+
+# Standalone KV Indexer
+
+## Overview
+
+The standalone KV indexer runs the KV cache radix tree as an independent service, separate from the router. It subscribes to KV events from workers, maintains a radix tree of cached blocks, and exposes a query endpoint (`kv_indexer_query`) that external clients can use to inspect or query KV cache state.
+
+This is distinct from the [Standalone Router](../../../../components/src/dynamo/router/README.md), which is a full routing service. The standalone indexer provides only the indexing and query layer without routing logic.
+
+## Use Cases
+
+- **Debugging**: Inspect the radix tree state to verify which blocks are cached on which workers.
+- **State verification**: Confirm that the indexer's view of KV cache state matches the router's internal state (used in integration tests).
+- **Custom routing**: Build external routing logic that queries the indexer for overlap scores and makes its own worker selection decisions.
+- **Monitoring**: Observe KV cache distribution across workers without running a full router.
+
+## API
+
+### Python
+
+```python
+from dynamo._internal import start_kv_block_indexer
+from dynamo.llm import KvRouterConfig
+
+# Start the standalone indexer on a component's endpoint
+await start_kv_block_indexer(endpoint, block_size, kv_router_config)
+```
+
+**Parameters:**
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `endpoint` | `Endpoint` | The Dynamo runtime endpoint on whose component the indexer will run |
+| `block_size` | `int` | KV cache block size (must match workers) |
+| `kv_router_config` | `KvRouterConfig` | Router configuration (controls event threading, TTL, etc.) |
+
+### Query Endpoint
+
+Once started, the indexer exposes a `kv_indexer_query` endpoint on the same component. Clients can send one of three request types:
+
+**`FindMatchesTokens`** — Given raw tokens, compute block hashes and return per-worker overlap scores:
+
+```python
+query_client = await query_endpoint.client()
+stream = await query_client.generate(
+    {"FindMatchesTokens": {"tokens": [1, 2, 3, ...], "block_mm_infos": None}},
+    annotated=False,
+)
+response = await stream.__anext__()
+# response == {"Matches": {"scores": {(worker_id, dp_rank): count, ...}, "frequencies": [...]}}
+```
+
+**`FindMatchesHashed`** — Same as above but with pre-computed block hashes:
+
+```python
+stream = await query_client.generate(
+    {"FindMatchesHashed": {"block_hashes": [hash1, hash2, ...]}},
+    annotated=False,
+)
+```
+
+**`DumpTree`** — Dump the full radix tree state as a list of router events:
+
+```python
+stream = await query_client.generate("DumpTree", annotated=False)
+response = await stream.__anext__()
+events = response["TreeDump"]  # List of RouterEvent objects
+```
+
+## Limitations
+
+- **JetStream not supported**: The standalone indexer does not support `durable_kv_events` (JetStream mode). It relies on NATS Core or ZMQ event plane with local indexer mode. Attempting to start with `durable_kv_events=True` will raise an error.
+- **No routing logic**: The indexer only maintains the radix tree and answers queries. It does not track active blocks, manage request lifecycle, or perform worker selection.
+
+## Architecture
+
+```mermaid
+graph TD
+    subgraph Workers
+        W1[Worker 1<br/>KvEventPublisher]
+        W2[Worker 2<br/>KvEventPublisher]
+    end
+
+    subgraph "Event Plane (NATS Core / ZMQ)"
+        EP[KV Events]
+    end
+
+    subgraph "Standalone Indexer"
+        SUB[Subscriber]
+        IDX[Indexer / Radix Tree]
+        QE[kv_indexer_query endpoint]
+    end
+
+    CLIENT[External Client]
+
+    W1 -->|publish events| EP
+    W2 -->|publish events| EP
+    EP -->|subscribe| SUB
+    SUB -->|apply events| IDX
+    CLIENT -->|FindMatches / DumpTree| QE
+    QE -->|query| IDX
+
+    style EP fill:#e1f5fe,stroke:#333,color:#333
+    style W1 fill:#f3e5f5,stroke:#333,color:#333
+    style W2 fill:#f3e5f5,stroke:#333,color:#333
+    style IDX fill:#2e8b57,stroke:#333,color:#fff
+    style SUB fill:#2e8b57,stroke:#333,color:#fff
+    style QE fill:#2e8b57,stroke:#333,color:#fff
+    style CLIENT fill:#fff3e0,stroke:#333,color:#333
+```
+
+The standalone indexer internally:
+
+1. Creates an `Indexer` instance with the given config and block size.
+2. Starts a subscriber that listens for KV events from workers via the event plane. On worker discovery, it queries the worker's local indexer to bootstrap state.
+3. Registers a `kv_indexer_query` endpoint that accepts `FindMatchesHashed`, `FindMatchesTokens`, and `DumpTree` requests.
+
+## See Also
+
+- **[Router Guide](router-guide.md)**: Full KV router configuration and tuning
+- **[Router Design](../../design-docs/router-design.md)**: Architecture and event transport modes
+- **[Standalone Router](../../../../components/src/dynamo/router/README.md)**: Full routing service (routes requests to workers)