Add HPL (LINPACK) benchmark as validate/stress task

HPL 2.3 from netlib compiled against OpenBLAS with a minimal
single-process MPI stub — no MPI package required in the ISO.
Matrix size is auto-sized to 80% of total RAM at runtime.

Build:
- VERSIONS: HPL_VERSION=2.3, HPL_SHA256=32c5c17d…
- build-hpl.sh: downloads HPL + OpenBLAS from Debian 12 repo,
  compiles xhpl with a self-contained mpi_stub.c
- build.sh: step 80-hpl, injects xhpl + libopenblas into overlay

Runtime:
- bee-hpl: generates HPL.dat (N auto from /proc/meminfo, NB=256,
  P=1 Q=1), runs xhpl, prints standard WR... Gflops output
- platform/hpl.go: RunHPL(), parses WR line → GFlops + PASSED/FAILED
- tasks.go: target "hpl"
- pages.go: LINPACK (HPL) card in validate/stress grid (stress-only)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build.sh

@@ -1148,6 +1148,19 @@ if [ "$BEE_GPU_VENDOR" = "nvidia" ]; then
     echo "=== john injected ==="
 fi
 
+# --- build HPL (CPU LINPACK) — runs on all variants ---
+run_step "build HPL ${HPL_VERSION}" "80-hpl" \
+    sh "${BUILDER_DIR}/build-hpl.sh" "${HPL_VERSION}" "${HPL_SHA256}" "${DIST_DIR}"
+
+HPL_CACHE="${DIST_DIR}/hpl-${HPL_VERSION}"
+mkdir -p "${OVERLAY_STAGE_DIR}/usr/local/lib/bee"
+cp "${HPL_CACHE}/bin/xhpl" "${OVERLAY_STAGE_DIR}/usr/local/lib/bee/xhpl"
+chmod +x "${OVERLAY_STAGE_DIR}/usr/local/lib/bee/xhpl"
+chmod +x "${OVERLAY_STAGE_DIR}/usr/local/bin/bee-hpl" 2>/dev/null || true
+# Inject OpenBLAS runtime libs needed by xhpl
+cp "${HPL_CACHE}/lib/"* "${OVERLAY_STAGE_DIR}/usr/lib/" 2>/dev/null || true
+echo "=== HPL injected: xhpl + $(ls "${HPL_CACHE}/lib/" | wc -l) OpenBLAS libs ==="
+
 # --- embed build metadata ---
 mkdir -p "${OVERLAY_STAGE_DIR}/etc"
 BUILD_DATE="$(date +%Y-%m-%d)"
@@ -1180,6 +1193,7 @@ BUILD_DATE=${BUILD_DATE}
 GIT_COMMIT=${GIT_COMMIT}
 DEBIAN_VERSION=${DEBIAN_VERSION}
 DEBIAN_KERNEL_ABI=${DEBIAN_KERNEL_ABI}
+HPL_VERSION=${HPL_VERSION}
 ${GPU_VERSION_LINE}
 EOF