docs: record --spec-default + Vulkan0 + PARALLEL=2 final state; 262k-ctx ROCm-vs-Vulkan bench

docs/frank-default-large-benchmark.md

@@ -6,20 +6,22 @@ llama.cpp build on that machine (upstream, b10133+).
 
 ## Summary
 
-- **Backend**: ROCm (HIP). Vulkan is not viable for the full pipeline.
-- **Spec (2026-08-15)**: DSpark sidecar draft (`draft-dspark`,
-  `magnitudedev/Qwen3.8-27B-DSpark-GGUF` Q8_0, 1.36B, `--spec-draft-n-max
-  7`). Replaced bundled MTP. **Single-slot (PARALLEL=1)** because DSpark
-  crashes the ROCm backend under two concurrent 16k prompts (see
+- **Backend**: **Vulkan0** (2026-08-15).  At 262k ctx Vulkan decodes 48%
+  faster than ROCm (5.68 vs 3.83 t/s); ROCm only wins prefill 119 vs 106
+  t/s.  Generation is the interactive cost, so Vulkan0.
+- **Spec (2026-08-15)**: llama.cpp `--spec-default` preset (ngram-mod,
+  n_match 24 / n_min 48 / n_max 64, draftless).  Replaced DSpark, which
+  crashed the ROCm backend under two concurrent 16k prompts (see
   [[qwen3.8-spec-matrix]]).
-- **Context**: 1 slot x 262144.
+- **Parallel**: 2 slots — passes the 2x32k parallel stream test.
+- **Context**: 2 slots x 262144.
 - **KV**: `q8_0` K / `q8_0` V on GPU.
 - **Batch**: 8192 / ubatch 256.
 
 > The A/B tables below are retained for history but predate the 2026-08-14
-> Qwen3.8-27B `UD-Q8_K_XL` switch to bundled MTP and the 2026-08-15 DSpark
-> switch.  The current production spec decision and the DSpark parallel crash
-> are recorded on [[qwen3.8-spec-matrix]].
+> Qwen3.8-27B `UD-Q8_K_XL` switch.  The current production spec decision,
+> the DSpark parallel crash, and the high-context ROCm-vs-Vulkan bench are
+> recorded on [[qwen3.8-spec-matrix]].
 
 ## 2026-08-14: Spec A/B (ROCm, production config, default sampling, 1.1k-token prompt)