rocm: implement the two DSpark kernels so --dspark can draft on gfx1151 (fixes #597)

[zom-2018]

Summary

Fixes #597 — "DSpark never produces a draft on ROCm / gfx1151".

Root cause: the DSpark engine path in ds4.c is backend-shared, but two GPU
kernels it needs existed only in ds4_cuda.cu. ROCm builds resolved them
through ROCM_UNAVAILABLE_INT stubs in ds4_rocm_unavailable.cu, so the
session probe always failed and every propose cycle ended with proposed=0.

This PR implements both kernels for ROCm (HIP) and removes the two stubs:

• dspark_markov_argmax_kernel + ds4_gpu_dspark_markov_argmax_tensor
  (rocm/ds4_rocm_indexer.cuh): on-device markov-chain argmax over the vocab
  with q8_0 w1/w2 rows — direct port of the CUDA kernel, same monotonic-key
  atomicMax tie-break.
• attention_noncausal_raw_batch_heads_kernel +
  ds4_gpu_attention_noncausal_raw_batch_heads_tensor
  (rocm/ds4_rocm_attention_launch.cuh): non-causal batch attention over the
  raw KV ring for draft blocks — direct port, same one-block
  max/denominator/value accumulation order; keeps the
  DS4_DSPARK_VERIFY_NONCAUSAL host-side oracle for the first 3 launches.

Signatures match the existing declarations in ds4_gpu.h; no other files are
touched.

Scope / safety

• ROCm-only change. No effect on CUDA, Metal, or CPU builds.
• No behavior change unless --dspark is used (the kernels are only reached
  through the DSpark propose/verify path).

Measured on AMD Strix Halo (Ryzen AI Max+ 395, Radeon 8060S gfx1151), ROCm 7.2

Before (upstream): proposed=0 every cycle (exactly the #597 symptom).

After, --dspark-confidence 0.3, 300 generated tokens, DeepSeek-V4-Flash-0731
abliterated GGUF (80.76 GiB) + its DSpark support drafter:

cycles=89 proposed=216 accepted_draft=206 accept_rate=95.37% avg_accept=2.315
full=39 partial=5 draft_len_hist=1:8,2:5,3:1 accepted_len_hist=0:118,1:10,2:3,3:1

At the default --dspark-confidence 0.9 the adaptive scheduler stays
conservative: proposed=21, accept_rate=90.5%.

Wall-clock is currently break-even on this bandwidth-bound APU (the 6-row
target verify costs ~4–5x a single decode pass, dominated by routed-expert
weight reads), so this is not a speedup claim — it makes DSpark functional
on ROCm, which is the precondition for any follow-up verify-path optimization.

Testing

• make strix-halo clean (gfx1151).
• CLI generation with --dspark + DS4_DSPARK_STATS=1: drafts proposed,
  verified and accepted; coherent output; stats counters advance.
• Without --dspark: unchanged behavior.