refactor: introduce Purchase Invoice GL composer

Phase 3 of the accounts/controller refactor. Adds PurchaseInvoiceGLComposer;
PI's get_gl_entries body moves into compose() and the method becomes a thin
shim. Row-builder methods still live on the document (invoked via self.doc)
and migrate onto the composer next.

After comparing the SI and PI compose() flows, BaseGLComposer is kept minimal:
the two differ in step order, builders, and per-doctype regional function, so a
shared template is not warranted. No behavior change (Phase 0 snapshots and PI
GL tests stay green).

erpnext/accounts/doctype/purchase_invoice/purchase_invoice.py

@@ -856,34 +856,11 @@ class PurchaseInvoice(BuyingController):
 			)
 
 	def get_gl_entries(self, inventory_account_map=None):
-		self.auto_accounting_for_stock = erpnext.is_perpetual_inventory_enabled(self.company)
+		from erpnext.accounts.doctype.purchase_invoice.services.gl_composer import (
+			PurchaseInvoiceGLComposer,
+		)
 
-		if self.auto_accounting_for_stock:
-			self.stock_received_but_not_billed = self.get_company_default("stock_received_but_not_billed")
-		else:
-			self.stock_received_but_not_billed = None
-
-		self.negative_expense_to_be_booked = 0.0
-		gl_entries = []
-
-		self.make_supplier_gl_entry(gl_entries)
-		self.make_item_gl_entries(gl_entries)
-		self.make_precision_loss_gl_entry(gl_entries)
-
-		self.make_tax_gl_entries(gl_entries)
-		self.make_internal_transfer_gl_entries(gl_entries)
-		self.make_gl_entries_for_tax_withholding(gl_entries)
-
-		gl_entries = make_regional_gl_entries(gl_entries, self)
-
-		gl_entries = merge_similar_entries(gl_entries)
-
-		self.make_payment_gl_entries(gl_entries)
-		self.make_write_off_gl_entry(gl_entries)
-		self.make_gle_for_rounding_adjustment(gl_entries)
-		self.set_transaction_currency_and_rate_in_gl_map(gl_entries)
-		self.set_gl_entry_for_purchase_expense(gl_entries)
-		return gl_entries
+		return PurchaseInvoiceGLComposer(self).compose(inventory_account_map)
 
 	def check_asset_cwip_enabled(self):
 		# Check if there exists any item with cwip accounting enabled in it's asset category

erpnext/accounts/doctype/purchase_invoice/services/gl_composer.py

@@ -0,0 +1,48 @@
+# Copyright (c) 2015, Frappe Technologies Pvt. Ltd. and Contributors
+# License: GNU General Public License v3. See license.txt
+
+import erpnext
+from erpnext.accounts.services.base_gl_composer import BaseGLComposer
+
+
+class PurchaseInvoiceGLComposer(BaseGLComposer):
+	"""Assembles the GL entries for a Purchase Invoice.
+
+	Orchestration only for now: the voucher-specific row builders still live on
+	the Purchase Invoice document and are invoked via ``self.doc``. They migrate
+	onto this composer in a later increment.
+	"""
+
+	def compose(self, inventory_account_map=None):
+		from erpnext.accounts.doctype.purchase_invoice.purchase_invoice import make_regional_gl_entries
+		from erpnext.accounts.general_ledger import merge_similar_entries
+
+		doc = self.doc
+		doc.auto_accounting_for_stock = erpnext.is_perpetual_inventory_enabled(doc.company)
+
+		if doc.auto_accounting_for_stock:
+			doc.stock_received_but_not_billed = doc.get_company_default("stock_received_but_not_billed")
+		else:
+			doc.stock_received_but_not_billed = None
+
+		doc.negative_expense_to_be_booked = 0.0
+		gl_entries = []
+
+		doc.make_supplier_gl_entry(gl_entries)
+		doc.make_item_gl_entries(gl_entries)
+		doc.make_precision_loss_gl_entry(gl_entries)
+
+		doc.make_tax_gl_entries(gl_entries)
+		doc.make_internal_transfer_gl_entries(gl_entries)
+		doc.make_gl_entries_for_tax_withholding(gl_entries)
+
+		gl_entries = make_regional_gl_entries(gl_entries, doc)
+
+		gl_entries = merge_similar_entries(gl_entries)
+
+		doc.make_payment_gl_entries(gl_entries)
+		doc.make_write_off_gl_entry(gl_entries)
+		doc.make_gle_for_rounding_adjustment(gl_entries)
+		doc.set_transaction_currency_and_rate_in_gl_map(gl_entries)
+		doc.set_gl_entry_for_purchase_expense(gl_entries)
+		return gl_entries