feat(kv-router): host-supplied inputs for SelectionService parity with the frontend scheduler

[ishandhanani]

PR 1 of 9 implementing ai-dynamo#14310 (one SelectionCore for the frontend, the standalone selection service, and the EPP). Base: main.

Fork staging copy of the upstream PR, rebased onto ai-dynamo/main. Replaces fork #8 to #12, which stay open for review history.

Summary

Adds host-supplied scheduling inputs to SelectionCore so the frontend can later embed the same scheduler used by the standalone service. The core gains the frontend's load, eligibility, shared-cache and session inputs without depending on dynamo-runtime.

Implementation

• SelectionHostHooks supplies the prefill estimator, overload and availability providers, shared KV cache and LoRA filter to each partition.
• Shared-cache lookup runs alongside primary-index lookup, and structured session context takes precedence over the legacy flat session ID.
• A common LoRA narrowing helper preserves the caller's allow-set and pinned-worker semantics.
• A reservation index resolves lifecycle operations by selection ID before falling back to partition scanning for replica-created bookings.
• Advisory selection reads projected worker load without queue admission or booking; it cannot be used to reserve a worker.

Walkthrough

flowchart LR
  Host[Host inputs] --> Core[SelectionCore]
  Request[Selection request] --> Core
  Core --> Partition[Partition scheduler]
  Partition --> Decision[Worker and projected load]

Loading

Inputs and precedence

prepare_selection_inputs combines indexed overlap, optional shared-cache hits, caller constraints and host eligibility before scheduling. A failed shared-cache lookup logs the failure and proceeds without those hits. LoRA narrowing never widens the caller's allow-set; an allowed pin survives, and an empty narrowing falls back to the caller's set.

SelectionSessionContext carries session_id, parent_session_id, session_final and input_trigger. The trigger maps to the current typed selector contract. KV hints are not a field of this structured session object.

Reservation lifecycle

Both booking paths populate the selection-ID-to-partition index. Prefill completion, output-block updates and free operations consult it first. Partition scanning remains necessary for bookings received from replica peers. A lazily started sweep removes entries after out-of-band booking release; construction itself does not require a Tokio runtime.

Stack boundary

Hooks default to absent, so existing standalone callers retain their prior inputs. This PR supplies the scheduling interface; frontend embedding follows in #47. The hook fields are regrouped into SelectionHost in #49, and reservation-owned affinity leases are added in #53.

Validation

Current stack boundary: 888548e387e778baa5b3df123af9a1547899ea4e; the series is based on Dynamo main ab58558c2dff28f4ff1ea4e219030eac5da3f48f.

• Passed cargo check -p dynamo-kv-router -p dynamo-llm -p dynamo-ext-proc --no-default-features --features dynamo-kv-router/standalone-selection,dynamo-kv-router/standalone-slot-tracker --all-targets for this stack boundary.
• The complete series was tested at feat(kv-router): session affinity for the standalone service and EPP #53; its description records the Rust/Python suites, aggregated GPU correctness probes and paired main/final benchmarks. Those end-to-end results apply to the final stack, not to a separate deployment of this intermediate PR.

GitHub CI is not green across this series. Recurring failures are the missing fork Fastchecker runner group, helm-tester image and approval-bot token, plus published-docs composition and an unchanged external documentation link. The local compilation and final-stack validation above are recorded separately from those CI failures.

Stack

#	Branch	Title	Fork PRs it replaces
1 (this PR)	idhanani/up-1-host-hooks	feat(kv-router): host-supplied inputs for SelectionService parity with the frontend scheduler	#8 to #12
2	idhanani/up-2-core-indexing	feat(kv-router): approximate, remote, and hint-aware indexing in SelectionService	#14 to #16
3	idhanani/up-3-embed-core	feat(router): re-host KvRouter scheduling on an embedded selection partition	#18, #35
4	idhanani/up-4-delete-kvscheduler	refactor(router): delete the runtime-bound KV router scheduler	#36
5	idhanani/up-5-policy-source	refactor(router): selection policy source, SelectionHost, and dead-flag removal	#37
6	idhanani/up-6-membership	feat(kv-router): one worker-membership path for frontend and EPP	#38
7	idhanani/up-7-kv-index-source	refactor(kv-router): one Indexer type and one KV-event ingress for every selection host	#39, #41, #43
8	idhanani/up-8-affinity-table	refactor(kv-router): one session-affinity implementation for every selection host	#42
9	idhanani/up-9-epp-affinity	feat(kv-router): session affinity for the standalone service and EPP	#44