feat(billing): 2d dual-run reconciliation (Odoo-computed vs NexaCloud-actual)

fusion.billing.reconciliation gains the compute: _compute_reconciliation
(flat + charge overage vs external, status match/delta at a tolerance) and
_reconcile_rows (resolve shadow sub -> flat + charge, upsert one row per
service/partner/period, per-row isolated). The wizard gains a read-only
_read_reconciliation_rows (NexaCloud usage cpu_hours*3600 + invoice-item
subtotals per YYYY-MM) and a "Run Reconciliation" button. 2a amended to
stamp x_fc_nexacloud_plan_id on shadow subs so reconciliation can find the
charge. Read-only on NexaCloud; writes only reconciliation rows (shadow
guarantees intact). 8 new tests, full suite green on odoo-trial.

fusion_centralize_billing/models/sale_order.py

@@ -11,6 +11,9 @@ class SaleOrder(models.Model):
         index=True, copy=False,
         help="Source NexaCloud subscription id — the importer's idempotency key.")
     x_fc_nexacloud_deployment_id = fields.Char(index=True, copy=False)
+    x_fc_nexacloud_plan_id = fields.Char(
+        index=True, copy=False,
+        help="Source NexaCloud plan id — links the shadow sub to its charge for 2d reconciliation.")
     x_fc_billing_service_id = fields.Many2one(
         "fusion.billing.service", index=True, copy=False, ondelete="set null")
     x_fc_shadow = fields.Boolean(