fix StockManager lot/serial coercion + add document flow test suite

- StockManager: coerce lot_number/serial_number to trimmed string (fixes
  Array-to-string warnings); validate quantity > 0 on insert; tighten
  transfer pair lookup to match in/out side by column value
- PostingWindowGuard: strip time component from datetime strings before
  date-format validation
- docs/tests/doc_flow_test.php: 32-assertion document flow suite covering
  Stock In create + approve, Sales Order draft/confirm, Invoice from Order
  (with duplicate-block check), PO create/confirm, Quotation create, and
  usage increment wiring check; all 32/32 PASS against wms_codex_test

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

app/assets/utils/classes/StockManager.php

@@ -233,7 +233,12 @@ class StockManager {
     public function saveStockIn(array $data, array $logging, string $uuid): int
     {
         $id           = (int)($data['id']        ?? 0);
-        $warehouse_id = (int)($data['warehouse'] ?? 0);
+        $warehouse_id = (int)($data["warehouse"] ?? 0);
+        $quantity     = (float)($data['quantity'] ?? 0);
+
+        if ($id === 0 && $quantity <= 0) {
+            throw new Exception("Quantity must be greater than zero.");
+        }
         $whMgmt       = new WarehouseManager($this->pdo, $this->company_id);
 
         $ctx       = $whMgmt->getStockContext($warehouse_id, $id);
@@ -267,8 +272,8 @@ class StockManager {
 
         } else {
 
-            $lot_number  = $data['lot_number']  ?: null;
-            $expiry_date = $data['expiry_date'] ?: null;
+            $lot_number  = trim((string)($data["lot_number"] ?? ""));
+            $expiry_date = trim((string)($data["expiry_date"] ?? ""));
 
             // Upsert md_lot: preserve existing expiry_date if already recorded
             if ($lot_number && $expiry_date) {
@@ -296,7 +301,7 @@ class StockManager {
                 ':company_id'    => $this->company_id,
                 ':date'          => date('Y-m-d H:i:s'),
                 ':product_sku'   => $data['product_sku'],
-                ':quantity'      => $data['quantity'],
+                ':quantity'      => $quantity,
                 ':price'         => (float)($data['price'] ?? 0),
                 ':zone'          => $data['zone'],
                 ':aisle'         => $data['aisle'],
@@ -305,7 +310,7 @@ class StockManager {
                 ':description'   => $data['description'] ?? '',
                 ':log'           => json_encode($table_log),
                 ':lot_number'    => $lot_number,
-                ':serial_number' => ($data['serial_number'] ?? null) ?: null,
+                ':serial_number' => trim((string)($data["serial_number"] ?? "")),
             ]);
 
             $td_stock_id = (int)$this->pdo->lastInsertId();
@@ -788,6 +793,7 @@ class StockManager {
             $sth = $this->pdo->prepare(
                 "SELECT * FROM `{$paired_table}`
                  WHERE uuid = :uuid AND company_id = :company_id
+                   AND `in` > 0
                  LIMIT 1"
             );
             $sth->execute([':uuid' => $row['uuid'], ':company_id' => $this->company_id]);
@@ -798,6 +804,7 @@ class StockManager {
                 $sth = $this->pdo->prepare(
                     "SELECT * FROM `{$from_table}`
                      WHERE uuid = :uuid AND company_id = :company_id
+                       AND `out` > 0
                      LIMIT 1"
                 );
                 $sth->execute([':uuid' => $row['uuid'], ':company_id' => $this->company_id]);