frank-default-large: re-enable --flash-attn on via QSA F32-mask patch

fa on aborted graph build: build_attn_qsa called ggml_fill on the
F16-converted kq mask (GGML_ASSERT F32 at ggml.c:5371). Patch mirrors the
MLA sparse path in llm_graph_context::build_attn (F32 base mask for the
top-k surgery, F16 cast only at the attention call, gated on
cparams.flash_attn). Vendored at scripts/patches/qwen4exp-qsa-f32-mask.patch
(sha256 f2f4a486...); the build-qwen38 binary on Dankclaw is rebuilt with
it and validated live at CTX_SIZE=32768 (health 200, real completion).

scripts/serve-frank-large.sh

@@ -59,7 +59,7 @@ fi
 : "${SPLIT_MODE:=none}"
 : "${FIT:=off}"
 : "${NGL:=999}"
-: "${FLASH_ATTN:=off}"
+: "${FLASH_ATTN:=on}"
 : "${TEMPERATURE:=0.7}"
 : "${TOP_K:=20}"
 : "${TOP_P:=0.8}"