cpustress+orchestrator: serial CPU/RAM passes + silent-skip guard

Orion's run (log 20:49 → 20:54) shipped GREEN while silently skipping
CPUStress. Two compounding bugs:

1. CPUStress ran --cpu N AND --vm N --vm-bytes 90% concurrently.
   On a 4-core 8 GiB N95, that's 360% RAM overcommit; the OOM-killer
   fired, usually on the agent itself. Replaced with two sequential
   passes — CPU (all methods, --verify) for 3 min, then RAM (--vm 1,
   --vm-bytes capped to MemAvailable − 1.5 GiB, floor 256 MiB, --verify)
   for 3 min. Each pass now also asserts elapsed ≥ target − 2s so a
   premature clean exit counts as failure instead of a silent pass.

2. On systemd-restart after the OOM, the agent hardcoded nextStage :=
   "Inventory" and re-ran it. The orchestrator's /result handler
   advances run state via TriggerStageCompleted against the *current*
   RunState, not against body.Stage — so an Inventory result posted
   while the run was in StateCPUStress silently advanced CPUStress →
   Storage and marked CPUStress passed without it ever running.

Two-layer defense for #2:
- agent-side: /claim response now carries current_state; agent resumes
  at the matching stage on a re-claim (happy path).
- server-side: new TriggerStageMismatch + StageNameForState helper
  backstop. If body.Stage doesn't match the run's current stage, /result
  parks the run in FailedHolding with failed_stage labeled
  "<got> (expected <expected>)" and returns 409.

Other stages audited for similar unbounded concurrency — none found;
only CPUStress was unsafe.

Tests:
- cpustress_test.go — parseMemAvailable parses real meminfo, errors on
  missing/malformed; cap calc hits floor on tiny boxes, uses 1.5 GiB
  headroom on normal/huge boxes.
- statemachine_test.go — TriggerStageMismatch lands at FailedHolding
  from every stage state and is rejected from pre-stage/terminal
  states; StageNameForState round-trips the stageStates map.
- agent_handlers_test.go — TestResult_RejectsMismatchedStage proves
  the Orion scenario now 409s + FailedHolding; TestResult_AcceptsMatchingStage
  proves the guard doesn't break the happy path.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

agent/tests/cpustress_test.go

@@ -0,0 +1,88 @@
+package tests
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestParseMemAvailable_RealSample exercises parseMemAvailable on a
+// real /proc/meminfo snippet. Units are always kB and always the
+// second field; we just want to confirm we strip it correctly.
+func TestParseMemAvailable_RealSample(t *testing.T) {
+	sample := `MemTotal:        8053292 kB
+MemFree:         3205104 kB
+MemAvailable:    6742180 kB
+Buffers:          145332 kB
+Cached:          2934064 kB
+`
+	got, err := parseMemAvailable(strings.NewReader(sample))
+	if err != nil {
+		t.Fatalf("parseMemAvailable: %v", err)
+	}
+	want := int64(6742180) * 1024
+	if got != want {
+		t.Errorf("MemAvailable = %d bytes, want %d", got, want)
+	}
+}
+
+func TestParseMemAvailable_Missing(t *testing.T) {
+	sample := "MemTotal:        8053292 kB\nMemFree:         3205104 kB\n"
+	if _, err := parseMemAvailable(strings.NewReader(sample)); err == nil {
+		t.Errorf("expected error when MemAvailable absent")
+	}
+}
+
+func TestParseMemAvailable_Malformed(t *testing.T) {
+	sample := "MemAvailable:\n"
+	if _, err := parseMemAvailable(strings.NewReader(sample)); err == nil {
+		t.Errorf("expected error on single-field MemAvailable line")
+	}
+}
+
+// TestMemCap_Normal: on a healthy 8GiB box with ~6.4GiB available,
+// cap lands at ~4.9GiB — well above floor, well below total.
+func TestMemCap_Normal(t *testing.T) {
+	avail := int64(6742180) * 1024 // ~6.4 GiB
+	cap := avail - memHeadroomBytes
+	if cap < memFloorBytes {
+		t.Errorf("cap=%d should be ≥ floor=%d on 6.4GiB available", cap, memFloorBytes)
+	}
+	// Sanity: headroom carved off 1.5 GiB.
+	if got := avail - cap; got != memHeadroomBytes {
+		t.Errorf("headroom = %d, want %d", got, memHeadroomBytes)
+	}
+}
+
+// TestMemCap_FloorHit: a box with <1.75 GiB available should fall
+// below the floor so CPUStress refuses the memory pass rather than
+// running a window too small to be meaningful.
+func TestMemCap_FloorHit(t *testing.T) {
+	avail := int64(1_500_000_000) // 1.4 GiB
+	cap := avail - memHeadroomBytes
+	if cap >= memFloorBytes {
+		t.Errorf("cap=%d should be < floor=%d on 1.4GiB available (cap pre-clamp)", cap, memFloorBytes)
+	}
+}
+
+// TestMemCap_HugeBox: a 128 GiB box still honors the 1.5 GiB
+// headroom (no weird upper clamp that would cap us at a tiny
+// fraction of the RAM).
+func TestMemCap_HugeBox(t *testing.T) {
+	avail := int64(128) * 1024 * 1024 * 1024
+	cap := avail - memHeadroomBytes
+	if cap < avail-2*memHeadroomBytes {
+		t.Errorf("cap=%d unexpectedly below avail=%d − 2×headroom", cap, avail)
+	}
+	// Should be comfortably above 100 GiB.
+	if cap < 100*1024*1024*1024 {
+		t.Errorf("cap=%d should exceed 100 GiB on 128 GiB box", cap)
+	}
+}
+
+// TestDurationSeconds_BelowOne floors at "1s"; stress-ng rejects 0.
+func TestDurationSeconds_BelowOne(t *testing.T) {
+	got := durationSeconds(0)
+	if got != "1s" {
+		t.Errorf("durationSeconds(0) = %q, want 1s", got)
+	}
+}