feat(gms): add host-tier storage contracts [GMS 06/18]

[hutm]

Summary

Define the engine-neutral contracts for moving KV content between GPU memory, host memory, and durable storage.

Scope

Adds ring, memory, storage, and transfer interfaces only. Daemon lifecycle, engine adapters, and optional network backends remain separate.

Stack

Position 6/18 in the GMS-managed KV review train. This PR is based on review/gms-v2-latehost-04-sglang-integration.

Parent: #10450
Tracking: #10455

Review migration

This is the current review entry replacing historical merged PR #10472. GitHub does not allow a merged PR to be retargeted; #10472 and all of its comments and reviews remain intact as the historical record.

The train is rebased on main at aeda23b483831df2f75b697b8ccf65f4ffd9f3d6. The reordered functionality is preserved while each PR boundary is independently buildable and lintable: compatibility call sites and the engine-facing placement adapter now appear at first use; missing type imports and test markers were added; repository-pinned formatting was applied; one unused constant was removed; and every commit carries a DCO sign-off. The complete range passes git diff --check and the full pre-commit suite.

Validation

Contract and component tests are included with the host-tier implementation; cross-node behavior is completed in positions 8-9.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[hutm]

Closing this PR because the deprecated umbrella #10450 has been replaced by the deliberately vLLM-only MVP tracker #11600. This PR covers work outside that MVP and is deferred to a future milestone; it is not being marked implemented, rejected, or unnecessary. The branch, comments, and reviews are intentionally retained so future work can reference the design and discussion.