Fix PSU slot regex: match MSI underscore format PSU1_POWER_IN

\b does not fire between a digit and '_' because '_' is \w in RE2.
The pattern \bpsu?\s*([0-9]+)\b never matched PSU1_POWER_IN style
sensors, so parsePSUSDR (and PSUSlotsFromSDR / samplePSUPower) returned
empty results for MSI servers — causing all power graphs to fall back
to DCMI which reports ~half actual draw.

Added an explicit underscore-terminated pattern first in the list and
tests covering the MSI format.

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

audit/internal/collector/psu.go

@@ -160,6 +160,9 @@ type psuSDR struct {
 }
 
 var psuSlotPatterns = []*regexp.Regexp{
+	// MSI/underscore style: PSU1_POWER_IN, PSU2_POWER_OUT — underscore is \w so \b
+	// does not fire after the digit; match explicitly with underscore terminator.
+	regexp.MustCompile(`(?i)\bpsu([0-9]+)_`),
 	regexp.MustCompile(`(?i)\bpsu?\s*([0-9]+)\b`),                    // PSU1, PS1, ps 2
 	regexp.MustCompile(`(?i)\bps\s*([0-9]+)\b`),                      // PS 6, PS6
 	regexp.MustCompile(`(?i)\bpws\s*([0-9]+)\b`),                     // PWS1

audit/internal/collector/psu_sdr_test.go

@@ -49,6 +49,10 @@ func TestParsePSUSlotVendorVariants(t *testing.T) {
 		{name: "PWS1 Status", want: 1},
 		{name: "Power Supply Bay 8", want: 8},
 		{name: "PS 6 Input Power", want: 6},
+		// MSI underscore format — \b does not fire between digit and '_'
+		{name: "PSU1_POWER_IN", want: 1},
+		{name: "PSU2_POWER_OUT", want: 2},
+		{name: "PSU4_STATUS", want: 4},
 	}
 
 	for _, tt := range tests {
@@ -59,6 +63,31 @@ func TestParsePSUSlotVendorVariants(t *testing.T) {
 	}
 }
 
+func TestParsePSUSDRMSIFormat(t *testing.T) {
+	t.Parallel()
+	raw := `
+PSU1_STATUS      | F1h | ok
+PSU1_POWER_OUT   | 928 Watts | ok
+PSU1_POWER_IN    | 976 Watts | ok
+PSU2_STATUS      | F2h | ok
+PSU2_POWER_OUT   | 944 Watts | ok
+PSU2_POWER_IN    | 992 Watts | ok
+`
+	got := parsePSUSDR(raw)
+	if len(got) != 2 {
+		t.Fatalf("len(got)=%d want 2", len(got))
+	}
+	if got[1].inputPowerW == nil || *got[1].inputPowerW != 976 {
+		t.Fatalf("psu1 input power=%v want 976", got[1].inputPowerW)
+	}
+	if got[1].outputPowerW == nil || *got[1].outputPowerW != 928 {
+		t.Fatalf("psu1 output power=%v want 928", got[1].outputPowerW)
+	}
+	if got[2].inputPowerW == nil || *got[2].inputPowerW != 992 {
+		t.Fatalf("psu2 input power=%v want 992", got[2].inputPowerW)
+	}
+}
+
 func TestSynthesizePSUsFromSDR(t *testing.T) {
 	t.Parallel()