feat(gms): add cross-node host-tier orchestration [GMS 08/18]

[hutm]

Summary

Coordinate KV content discovery and transfer across nodes while keeping the storage and transport implementations interchangeable.

Scope

Adds cross-node transfer orchestration and validation helpers on top of the dedicated GMS KV cache manager. Optional NIXL and Mooncake implementations are deliberately deferred to position 15.

Stack

Position 8/18 in the GMS-managed KV review train. This PR is based on review/gms-v2-latehost-08-3-1-host-tier-engine-core.

Parent: #10450
Tracking: #10455 #10458

Review migration

This is the current review entry replacing historical merged PR #10480. GitHub does not allow a merged PR to be retargeted; #10480 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

• Includes host-tier validation and stress coverage; full engine E2E recipes are isolated in positions 10 and 13.
• The rewritten commit replayed cleanly on the updated GMS 07/18 boundary.
• Existing GMS 09-18 patches are exactly equivalent under git range-diff.

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[datadog-official]

⚠️ Warnings

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