feat(router): decode-first disaggregated coordination and catalog replay endpoint

[ishandhanani]

Stacked on #23 (idhanani/p4-zero-infra-docs). Roadmap: Phase 3 frontend wiring + Phase 4 catalog replay endpoint.

Summary

• Decode-first coordination in PrefillRouter (contract 5) behind --router-disagg-decode-first / DYN_ROUTER_DISAGG_DECODE_FIRST. The decode RoutingHost previews and books the decode worker first, the prefill request is constrained to that worker's KV-transfer domain via ModelManager::get_kv_transfer_routing_constraints, and decode is dispatched on the held RoutePlan after prefill completes. The conditional-disagg decision is evaluated on the same preview; a bypass runs prefill on the booked decode worker.
• Compensation policy --router-disagg-bypass-on-prefill-failure / DYN_ROUTER_DISAGG_BYPASS_ON_PREFILL_FAILURE: default frees the decode booking (Compensated) and fails the request; opt-in runs prefill on the decode worker (BypassAfterPrefillFailure). Every transition is logged with its LinkedBookingState.
• Fallback to the unchanged prefill-first path when the decode pool is not KV routed, the request pins a prefill worker, or the decode host is unavailable. RoutePlan::release is now a non-test method.
• Flags plumbed through KvRouterConfig (serde + env + compat struct), PyO3 KvRouterConfig constructor, .pyi, and the frontend argument group.
• Sidecar catalog registration now publishes the rehosted ZMQ replay_endpoint from GetKvEventSources (dp rank 0), so a standalone selection service can replay after a late subscribe.
• Docs: frontend-configuration.mdx param fields; standalone-selection.md describes the frontend wiring and the encoder pool as a third embedded instance (each per-role KvRouter starts its own partition under --router-embedded-selection; no extra coordinator needed).

Validation

• cargo check --workspace --all-targets and cargo check in lib/bindings/python: clean.
• cargo clippy -p dynamo-llm -p dynamo-kv-router -p dynamo-vllm-sidecar --all-targets: no warnings.
• cargo test -p dynamo-llm prefill_router: 36 passed, including new decode_first_falls_back_to_prefill_first_without_a_decode_host and decode_first_plan_is_none_when_a_prefill_worker_is_pinned.
• cargo test -p dynamo-kv-router --features standalone-selection disagg_coordination_flags: passed.
• cargo test -p dynamo-vllm-mocker sidecar_registers: passed (catalog registration with replay endpoint field).
• Python arg group parses --router-disagg-decode-first --router-disagg-bypass-on-prefill-failure (both true; defaults false).
• docs_lint.py: 0 errors.

Not validated end to end against a real prefill+decode deployment with KV-transfer topology; the flag defaults to off.

[ishandhanani]

Closed while Stack 1 (#45 to #53) and Stack 2 (#40) go upstream; the branch stays for later stacks (direct dispatch, catalog leases, runtime features). See #33.