fix(router): backport host-controlled affinity APIs to release/1.6.0

[Harrilee]

Summary

Backport merged #15631 to release/1.6.0 so the release can expose the native routing interfaces required by AISimulate #306. The inspected release branch predates SessionAffinity::with_manual_clock; publishing it unchanged would not provide the API that AISimulate uses.

This is an unmodified cherry-pick -x of 094aaca91941421bb0dcb645b0f700a42d6de3ef, with no new routing design. AISimulate still owns AgentX simulation and calls Dynamo's production policy. Dynamo #15240 remains optional compatibility cleanup.

Details

The backport preserves #15631's host-controlled affinity clock, shared group-key helper, and queue capacity-recheck entry point, together with its original callers/tests. It does not publish packages or change release automation.

AISimulate's release-artifact check currently fails because the required native policy packages are absent from crates.io. The registry dependency closure at 1.6.0 is dynamo-tokens, dynamo-kv-hashing, dynamo-truthy, dynamo-runtime, dynamo-kv-router, and dynamo-custom-policy-builtin; Cargo must resolve optional registry dependencies when packaging too. After this backport is accepted, the matching packages still need the normal Dynamo release/DevOps staging and promotion process. Merging this PR alone does not unblock AISimulate's artifact check.

Validation

• The four-file patch matches the original feat(router): support host-controlled affinity time #15631 patch exactly and applies without conflicts to release commit 04720e4d.
• Native affinity tests: 10 passed. Policy queue tests: 24 passed.
• AISimulate now reuses Dynamo's public NoopSequencePublisher (AISimulate commit 6f0cdd2d43ece4496b1267e7c8095fd1d007df26), avoiding the main-only LocalWorkerLoad callback signature. Its 42 targeted native-router tests pass with the existing main pin.
• An isolated copy of that AISimulate source passes cargo check --offline --lib --features native-dynamo-router with its actual native dependency graph redirected to this release/backport source. The production Git pin and lockfile were not changed. No additional metrics backport is required.
• Packages have not been published; hosted CI is not asserted.

Where should the reviewer start?

Compare the patch with #15631, then review lib/kv-router/src/services/selection/affinity.rs and lib/kv-router/src/scheduling/policy_queue.rs against the release branch. Verify that the release source and promoted crate versions contain these APIs together.

Related Issues

Relates to AIC-1817, original #15631, and AISimulate #306.