Close remaining heterogeneous recurrent-state runtime gaps

[justinchuby]

Current released-runtime behavior

ORT GenAI 0.15.2 already discovers more heterogeneous decoder state than this issue previously credited:

• DefaultKeyValueCache discovers sparse/nonconsecutive KV layer indices from graph inputs.
• RecurrentState discovers past_key_values.<layer>.conv_state inputs and derives a companion past_key_values.<layer>.recurrent_state name.

The blocker is therefore not a blanket inability to describe mixed KV and recurrent slots.

Precise remaining gaps

1. Recurrent-state naming contract: Mobius hybrid exports currently use past_key_values.<layer>.ssm_state, while ORT GenAI 0.15.2 derives and binds past_key_values.<layer>.recurrent_state from each discovered conv_state input. Nemotron-H/Jamba packages cannot bind that state without renaming one side or making the runtime suffix configurable.
2. Beam reorder: DecoderOnly_State::UpdateInputsOutputs forwards beam indices to the KV cache update, but calls RecurrentState::Update() without beam indices. RecurrentState::Update() only swaps input/output buffers, so recurrent state is not reordered when beams are reordered.
3. Nonzero rewind: RecurrentState::RewindTo(index) rejects every index != 0. Stateful rollback to a retained nonzero sequence position is therefore unsupported.
4. Auxiliary graph topology remains separate work: MTP/NextN graphs still need explicit ownership, conditioning, and shared-weight relationships rather than being overloaded into KV metadata.

Tagged source evidence:

• recurrent_state.cpp at v0.15.2
• recurrent_state.h at v0.15.2
• decoder_only.cpp at v0.15.2
• kv_cache.cpp at v0.15.2

Acceptance criteria

1. A mixed attention + Mamba/Mamba2 package binds sparse KV, convolution, and recurrent inputs/outputs with exact names, shapes, dtypes, and global layer indices.
2. The recurrent suffix is compatible with Mobius ssm_state exports, either by a documented Mobius rename or a configurable/runtime-supported alias.
3. Recurrent state applies the same beam-index reorder semantics as KV state, with a test whose beam permutation changes the expected state rows.
4. Recurrent state supports nonzero rewind/rollback, with a test that rewinds to a retained index and then reproduces deterministic decode.
5. Prefill, cached decode, replay, reorder, and rollback pass for a tiny mixed-state package without treating recurrent state as KV cache.
6. A package with two MTP/NextN heads can represent graph ownership, conditioning outputs, and shared-weight relationships, or fails before durable package output with an actionable error.
7. Existing homogeneous decoder packages remain backward compatible.

Mobius continues to fail closed for released-runtime claims until these exact behaviors are proven.