docs: scope the OpenXLA SSM / hybrid / recurrent track (#502)

[inureyes]

Summary

Scoping design for Window D of epic #493: the SSM / hybrid / recurrent architecture track on the OpenXLA/IREE backend (Mamba/Mamba2, Jamba, Falcon-H1, RWKV7, Qwen3-Next, Plamo2, Nemotron-H, Kimi-Linear, RecurrentGemma). Design-doc only; no code changes.

What changed

• Add spike/openxla/SSM_HYBRID_DESIGN.md, matching the STAGE2_DESIGN.md convention. It grounds the track in the current codebase and covers the three deliverables the issue asks for:
  • The recurrent/state-space mixing primitives (selective scan / SSD, gated linear attention / delta rule, real-gated linear recurrent unit) and why they do not fit the shared attention core AttnLayout.
  • Explicit per-slot recurrent STATE allocation and carry through the continuous-batching engine, distinct from the KV cache: fixed-size (O(1), not growing with position), threaded as parallel device buffers via the same shim mechanism the KV cache already uses (kcache_b / vcache_b, xla_ensure_batch_kv).
  • Coexistence with KV-cache attention layers in hybrid models via a per-layer mixer dispatch in emit_transformer_layer, with a slot carrying both a KV region and a state region.
• Records an explicit decision to DEFER the implementation, with a staged plan.

Design decision

Deferred to a dedicated follow-up epic, #564, per the second option of the issue acceptance criteria. A first reference (Mamba2) requires a new scan graph primitive (whose prefill form is an unsolved lowering question), a new C-ABI per-slot state resource, a new per-layer mixer dispatch, and a new state-aware validation harness, comparable in size to the whole Stage 2 continuous-batching effort.

Test plan

• Documentation-only change; no source, build, or test surface touched.
• File references in the doc (AttnLayout, emit_ffn_body, moe_block, XlaBatchEngine, IreeRaggedLlama, xla_ensure_batch_kv, hybrid_ssm.rs, Mamba2Cache, kv_arch.rs) verified against the current tree.
• No AI attribution and no em dashes.

Closes #502