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