feat(kv-router): remote primary indexer for the selection service

[ishandhanani]

Phase 0 (selection-core parity), item 4b. Stacked on #14.

Summary

Ports the frontend Indexer::Remote shape (lib/llm/src/kv_router/indexer/mod.rs) into the runtime-free services backend, over HTTP instead of the Dynamo request plane. A selection service can serve its primary KV index from a standalone indexer, so several selector replicas (or a sidecar-backed deployment with no etcd/NATS) share one index and workers publish KV events to exactly one place.

• PrimaryRetention::Remote(Arc<RemoteIndexerTransport>), IndexerPolicy::with_remote_indexer(url) (validates scheme, trims trailing /). Rejected with use_kv_events=false: the standalone indexer has no routing-decision write path, so an approximate primary cannot be remote. A predict-on-route side indexer still layers locally.
• Indexer::Remote { primary: RemoteIndexerClient, approx }. Lookups call the standalone indexer's new POST /query_tiered_by_hash, which returns { block_size, tiered: WireTieredMatchDetails } (lossless device scores + lower-tier hits) rather than the Mooncake summary /query_by_hash renders. Local KV events are dropped with a trace log, remove_worker* only clears the side indexer, dump_events is empty (peers recover from the indexer), and transport/status/body failures map to KvRouterError::IndexerOffline. supports_router_hint_chain_retention() is false for remote (matches the frontend).
• SelectionCore.listens_for_kv_events = use_kv_events && !remote: with a remote primary, workers become schedulable without kv_events_endpoint(s), no ZMQ listener is registered or deregistered, and indexer_peers recovery is skipped with a warning.
• Plumbing: SelectionServiceBuilder::remote_indexer(url), SelectionServiceConfig::remote_indexer_url, --remote-indexer-url on python -m dynamo.select_service, and remote_indexer_url= on the Python SelectionService constructor.
• create_indexer_with_policy now takes the partition key (the remote client is per partition).
• Docs: standalone-selection.md (CLI row, indexing-modes row, remote semantics) and standalone-indexer.md (new endpoint).

Validation

• cargo test -p dynamo-kv-router --features standalone-selection --lib: 1067 passed. New: query_tiered_by_hash_returns_wire_tiered_matches (server round trip + 404 for unknown partition), remote_indexer_queries_the_standalone_service_and_merges_side_scores (live axum indexer on an ephemeral port: remote device + host-pinned hits, local side merge, dropped local events, empty dump, unknown partition and aborted server -> IndexerOffline), remote_indexer_rejects_approximate_primary_and_bad_urls, remote_indexer_serves_selection_without_local_kv_listeners (core admits workers without kv endpoints under use_kv_events=true and routes on remote cache credit).
• cargo check --features select-service in lib/bindings/python: clean (3m16s).
• cargo clippy --features standalone-selection --all-targets: only the pre-existing registry.rs dead-code warning. cargo fmt and docs_lint.py clean.
• Also folded into feat(kv-router): approximate and predict-on-route indexing in the selection service #14 (amended): its 2-thread side-indexer test now flushes Concurrent indexers, removing a race.

[ishandhanani]

Superseded by the upstream-shaped staging stack: continued as #46 (rebased onto ai-dynamo/main, see #33).