test(rocm): ReshapeAndCache->PagedAttention composition at real dims (issue #41)

[VikashLoomba]

Row

BACKEND-ROCM — test-hardening only (one additive cross-device case). Issue #41.

What changed

Adds a cross-device case for the KV-cache composition the in-tree suite doesn't cover: the existing paged-attention case hand-builds a contiguous cache, but the real model path writes KV via ReshapeAndCache and reads it back via PagedAttention. This case is that composition at real Qwen3.5-0.8B dims (Dh=256, Hq=8, Hkv=2, block_size 16), a shuffled block table, and a non-sequential slot mapping — the layout a stride/scatter bug would live in and the contiguous case cannot see.

Surfaced by the #41 Qwen3.5-0.8B divergence investigation: with this composition passing, every piece of the ROCm attention path validates in isolation, which is what localizes the residual divergence to bf16-softmax accumulation rather than a kernel defect (full causal chain in this #41 comment).

Evidence (4× gfx1100, ROCm 7.14, Release)

• New case: 7/7 vs the CPU oracle (runs on ROCm; the composition exercised end to end)
• Full cross-device suite green
• agent-preflight.sh --staged green; check-commit-trailers green

Speed claims

• This PR makes NO speed claim.

Honest gaps

• Test-only; no behavior change. The case validated the path as-is — it did not surface a defect (that was the point: it closes a coverage gap the divergence investigation needed ruled out).