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_ac/PostingWindowGuard.php

@@ -64,7 +64,11 @@ class PostingWindowGuard
             $date = "{$m[3]}-{$m[2]}-{$m[1]}";
         }
 
-        if (!preg_match('/^\d{4}-\d{2}-\d{2}$/', $date)) {
+        if (preg_match("#^([0-9]{4}-[0-9]{2}-[0-9]{2})[ T]#", $date, $m)) {
+            $date = $m[1];
+        }
+
+        if (!preg_match("#^[0-9]{4}-[0-9]{2}-[0-9]{2}$#", $date)) {
             throw new Exception("Invalid posting date {$date}. Use YYYY-MM-DD.");
         }