Deep dive: Account, Stock, Sale, Purchase — +800 LOC business logic

Account:
- Multi-currency: company_currency_id, amount_total_signed
- Lock dates on res.company (period, fiscal year, tax) + enforcement in action_post
- Recurring entries: account.move.recurring with action_generate (copy+advance)
- Tax groups: amount_type='group' computes child taxes with include_base_amount
- ComputeTaxes batch function, findTaxAccount helper

Stock:
- Lot/Serial tracking: enhanced stock.lot with expiration dates + qty compute
- Routes: stock.route model with product/category/warehouse selectable flags
- Rules: stock.rule model with pull/push/buy/manufacture actions + procure methods
- Returns: action_return on picking (swap locations, copy moves)
- Product tracking extension (none/lot/serial) + route_ids M2M

Sale:
- Pricelist: get_product_price with fixed/percentage/formula computation
- Margin: purchase_price, margin, margin_percent on line + order totals
- Down payments: action_create_down_payment (deposit invoice at X%)

Purchase:
- 3-way matching: action_create_bill now updates qty_invoiced on PO lines
- Purchase agreements: purchase.requisition + line with state machine

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

addons/account/models/account_account.go

@@ -109,6 +109,7 @@ func initAccountTax() {
 		orm.Boolean("include_base_amount", orm.FieldOpts{String: "Affect Base of Subsequent Taxes"}),
 		orm.Boolean("is_base_affected", orm.FieldOpts{String: "Base Affected by Previous Taxes", Default: true}),
 		orm.Many2many("children_tax_ids", "account.tax", orm.FieldOpts{String: "Children Taxes"}),
+		orm.Many2one("parent_tax_id", "account.tax", orm.FieldOpts{String: "Parent Tax Group"}),
 		orm.Char("description", orm.FieldOpts{String: "Label on Invoices"}),
 		orm.Many2one("tax_group_id", "account.tax.group", orm.FieldOpts{String: "Tax Group"}),
 		orm.Many2one("country_id", "res.country", orm.FieldOpts{String: "Country"}),