fix(exporter): assign PSU slot positions by serial order, not encounter order

disambiguatePSUSlots previously renumbered colliding PSU slots in whatever
order the devices happened to appear in after parsing/dedup, which isn't
guaranteed stable across runs. Sort each colliding group by serial number
ascending before assigning 0-based positions, so the same PSU always lands
in the same slot on re-conversion regardless of incidental ordering upstream.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/exporter/reanimator_converter.go

@@ -2188,26 +2188,29 @@ func dedupePSUs(items []ReanimatorPSU) []ReanimatorPSU {
 // string from the source. Some collectors (e.g. BEE-SP) report every PSU
 // at slot "0". A shared slot value prevents Reanimator from telling the
 // PSUs apart by installed position, so they land in "other components"
-// instead of being tracked as PSU installations. Reassigning stable 0-based
-// positions by encounter order is the best available disambiguation without
-// inventing a serial or other identity data we don't have.
+// instead of being tracked as PSU installations. Within each colliding
+// group, positions are assigned by ascending serial number rather than
+// source encounter order, so the result is deterministic and reproducible
+// across re-imports instead of depending on incidental parse ordering.
 func disambiguatePSUSlots(items []ReanimatorPSU) []ReanimatorPSU {
 	if len(items) < 2 {
 		return items
 	}
-	counts := make(map[string]int, len(items))
-	for _, it := range items {
-		counts[strings.ToLower(strings.TrimSpace(it.Slot))]++
+	groups := make(map[string][]int, len(items))
+	for i, it := range items {
+		key := strings.ToLower(strings.TrimSpace(it.Slot))
+		groups[key] = append(groups[key], i)
 	}
-	next := make(map[string]int, len(items))
-	for i := range items {
-		key := strings.ToLower(strings.TrimSpace(items[i].Slot))
-		if counts[key] < 2 {
+	for _, idxs := range groups {
+		if len(idxs) < 2 {
 			continue
 		}
-		idx := next[key]
-		next[key]++
-		items[i].Slot = strconv.Itoa(idx)
+		sort.Slice(idxs, func(a, b int) bool {
+			return strings.ToLower(items[idxs[a]].SerialNumber) < strings.ToLower(items[idxs[b]].SerialNumber)
+		})
+		for pos, idx := range idxs {
+			items[idx].Slot = strconv.Itoa(pos)
+		}
 	}
 	return items
 }