feat(ds4): add monolithic Strix Halo concurrent serving

[Graffioh]

Scope

This rebuilds the concurrency work for DeepSeek4 only. Qwen source and GGML kernels are unchanged.

It supersedes the experimental direction in #594, #595, #596 and Graffioh#16, while incorporating the MoE concurrency discussion. The supported topology is deliberately narrow: one local HIP gfx1151 Strix Halo with the complete model and all experts resident.

Implementation

• Adds a model-neutral continuous scheduler and SeqEngine boundary with admission, cancellation, fair prompt progress, slow-client isolation, and reusable slot lifecycle handling.
• Extends the shared paged allocator for reservations and rolling decode headroom.
• Adds DeepSeek4 persistent paged raw MLA, compressed MLA, and indexer state using 128-token model pages.
• Lowers up to 16 independent lanes into an exact gathered DeepSeek4 graph. Live decode rows share a weight pass; selected prompt rows advance one exact token per sequence in that same pass.
• Keeps sampling history and device block-table state isolated per reusable sequence slot.
• Auto-enables paged mode for --max-concurrency 2..16 and exposes optional physical-pool sizing through --kv-pool-tokens.

Guardrails

This path fails closed for CUDA, non-gfx1151 HIP devices, layer or remote target splits, DFLASH_DS4_MOE_TP, drafts/DSpark, DDTree, PFlash/KVFlash, fused decode, approximate prefill, windowed attention, and prefix-cache parking. It requires the full model to remain resident on the single Strix Halo.

Strix Halo concurrency screening

Model GGUF: DeepSeek-V4-Flash-ROCMFP2-STRIX.gguf
Device: one local HIP gfx1151 Strix Halo (Radeon 8060S), with the full model and experts resident.
Protocol: one fresh server and same-concurrency warm-up per level; short ragged, disjoint prompts; exact prefill; greedy streaming; 256 forced completion tokens per request; one repeat.

Concurrent requests	token/s
1	10.50 tok/s
8	25.88 tok/s
16	24.43 tok/s

No llama.cpp comparison is stated yet: the available llama-server rejects this ROCmFP GGUF at load time (invalid ggml type 101). A compatible llama.cpp/DeepSeek4 ROCmFP build (or a compatible GGUF) is required before publishing paired DS4-vs-llama results. This is a one-repeat screening, not a performance claim; use five paired repeats once that baseline is available.

Validation

• Built the HIP gfx1151 server and all focused concurrency targets, including test_deepseek4_unit.
• 358 selected scheduler, server, feature-gate, slot-manager, contract, page-layout, and paged-cache checks pass.
• Paged allocator suite: 13/13 pass.
• Model-backed DeepSeek4 screening completed at C=1, C=4, C=8, and C=16 (29/29 requests; 7,424/7,424 completion tokens; zero failures).
• Final publication commit is byte-identical to the validated worktree.
• Qwen directories, Qwen tests, and GGML kernels have no diff from upstream.