Rework Power Fit report: 90 min stability, aligned tables, PSU/fan sections

- Increase stability profile duration from 33 min to 90 min by wiring
  powerBenchDurationSec() into runBenchmarkPowerCalibration (was discarded)
- Collect per-step PSU slot readings, fan RPM/duty, and per-GPU telemetry
  in ramp loop; add matching fields to NvidiaPowerBenchStep/NvidiaPowerBenchGPU
- Rewrite renderPowerBenchReport: replace Per-Slot Results with Single GPU
  section, rework Ramp Sequence rows=runs/cols=GPUs, add PSU Performance
  section (conditional on IPMI data), add transposed Single vs All-GPU
  comparison table in per-GPU sections
- Add fmtMDTable helper (benchmark_table.go) and apply to all tables in
  both power and performance reports so columns align in plain-text view

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

audit/internal/platform/benchmark_table.go

@@ -0,0 +1,75 @@
+package platform
+
+import (
+	"strings"
+)
+
+// fmtMDTable renders a markdown table with column widths padded so the table
+// is readable as plain text without a markdown renderer.
+//
+// headers contains the column header strings.
+// rows contains data rows; each row must have the same number of cells as headers.
+// Cells with fewer entries than headers are treated as empty.
+func fmtMDTable(headers []string, rows [][]string) string {
+	ncols := len(headers)
+	if ncols == 0 {
+		return ""
+	}
+
+	// Compute max width per column.
+	widths := make([]int, ncols)
+	for i, h := range headers {
+		if len(h) > widths[i] {
+			widths[i] = len(h)
+		}
+	}
+	for _, row := range rows {
+		for i := 0; i < ncols; i++ {
+			cell := ""
+			if i < len(row) {
+				cell = row[i]
+			}
+			if len(cell) > widths[i] {
+				widths[i] = len(cell)
+			}
+		}
+	}
+
+	var b strings.Builder
+
+	// Header row.
+	b.WriteByte('|')
+	for i, h := range headers {
+		b.WriteByte(' ')
+		b.WriteString(h)
+		b.WriteString(strings.Repeat(" ", widths[i]-len(h)))
+		b.WriteString(" |")
+	}
+	b.WriteByte('\n')
+
+	// Separator row.
+	b.WriteByte('|')
+	for i := range headers {
+		b.WriteString(strings.Repeat("-", widths[i]+2))
+		b.WriteByte('|')
+	}
+	b.WriteByte('\n')
+
+	// Data rows.
+	for _, row := range rows {
+		b.WriteByte('|')
+		for i := 0; i < ncols; i++ {
+			cell := ""
+			if i < len(row) {
+				cell = row[i]
+			}
+			b.WriteByte(' ')
+			b.WriteString(cell)
+			b.WriteString(strings.Repeat(" ", widths[i]-len(cell)))
+			b.WriteString(" |")
+		}
+		b.WriteByte('\n')
+	}
+
+	return b.String()
+}