Avoid allocations when touching graph LRU entries

[LiangSu8899]

Summary

• move an existing graph key to the MRU position with std::list::splice
• preserve the insertion path for previously unseen keys
• add a focused allocation contract test

Why

Graph replay touches the selected shape key on every invocation. Erasing and reinserting an existing list node performs a host free/allocation pair in the hot path. Splicing the existing node preserves ordering without allocating.

Boundaries

• no ABI or exported symbol changes
• no graph selection, eviction, capture, or replay semantic changes
• no model- or provider-specific behavior
• new-key insertion remains allocation-permitted setup behavior

Validation

• Release exec build passed
• graph LRU allocation test passed
• 1,000 existing-key touches performed zero allocations
• unseen-key insertion remained functional and allocation-visible
• git diff --check passed