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>

audit/internal/webui/server_test.go

@@ -741,8 +741,8 @@ func TestBurnPageRendersGoalBasedNVIDIACards(t *testing.T) {
 	for _, needle := range []string{
 		`NVIDIA Max Compute Load`,
 		`dcgmproftester`,
-		`targeted_stress remain in <a href="/validate">Validate</a>`,
-		`NVIDIA Interconnect Test (NCCL all_reduce_perf)`,
+		`NCCL`,
+		`Validate → Stress mode`,
 		`id="burn-gpu-list"`,
 	} {
 		if !strings.Contains(body, needle) {