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/VERSIONS

@@ -19,5 +19,7 @@ ROCRAND_VERSION=3.2.0.60304-76~22.04
 HIP_RUNTIME_AMD_VERSION=6.3.42134.60304-76~22.04
 HIPBLASLT_VERSION=0.10.0.60304-76~22.04
 COMGR_VERSION=2.8.0.60304-76~22.04
+HPL_VERSION=2.3
+HPL_SHA256=32c5c17d22330e6f2337b681aded51637fb6008d3f0eb7c277b163fadd612830
 GO_VERSION=1.24.0
 AUDIT_VERSION=1.0.0