refactor: introduce GL composer and delegate SI get_gl_entries

Phase 2 (pilot) of the accounts/controller refactor. Adds BaseGLComposer
and SalesInvoiceGLComposer; Sales Invoice'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.
No behavior change (Phase 0 GL snapshots remain byte-identical).

erpnext/accounts/doctype/sales_invoice/sales_invoice.py

@@ -1576,38 +1576,9 @@ class SalesInvoice(SellingController):
 			make_reverse_gl_entries(voucher_type=self.doctype, voucher_no=self.name)
 
 	def get_gl_entries(self, inventory_account_map=None):
-		from erpnext.accounts.general_ledger import merge_similar_entries
+		from erpnext.accounts.doctype.sales_invoice.services.gl_composer import SalesInvoiceGLComposer
 
-		gl_entries = []
-
-		self.make_customer_gl_entry(gl_entries)
-
-		self.make_tax_gl_entries(gl_entries)
-		self.make_internal_transfer_gl_entries(gl_entries)
-
-		self.make_item_gl_entries(gl_entries)
-
-		disable_sdbnb_in_sr = frappe.get_cached_value("Company", self.company, "disable_sdbnb_in_sr")
-
-		if not (self.is_return and disable_sdbnb_in_sr):
-			self.stock_delivered_but_not_billed_gl_entries(gl_entries)
-
-		self.make_precision_loss_gl_entry(gl_entries)
-		self.make_discount_gl_entries(gl_entries)
-
-		gl_entries = make_regional_gl_entries(gl_entries, self)
-
-		# merge gl entries before adding pos entries
-		gl_entries = merge_similar_entries(gl_entries)
-
-		self.make_loyalty_point_redemption_gle(gl_entries)
-		self.make_pos_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)
-		return gl_entries
+		return SalesInvoiceGLComposer(self).compose(inventory_account_map)
 
 	def stock_delivered_but_not_billed_gl_entries(self, gl_entries):
 		if self.update_stock or not cint(erpnext.is_perpetual_inventory_enabled(self.company)):

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

@@ -0,0 +1,51 @@
+# Copyright (c) 2015, Frappe Technologies Pvt. Ltd. and Contributors
+# License: GNU General Public License v3. See license.txt
+
+import frappe
+
+from erpnext.accounts.services.base_gl_composer import BaseGLComposer
+
+
+class SalesInvoiceGLComposer(BaseGLComposer):
+	"""Assembles the GL entries for a Sales Invoice.
+
+	Orchestration only for now: the voucher-specific row builders still live on
+	the Sales 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.sales_invoice.sales_invoice import make_regional_gl_entries
+		from erpnext.accounts.general_ledger import merge_similar_entries
+
+		doc = self.doc
+		gl_entries = []
+
+		doc.make_customer_gl_entry(gl_entries)
+
+		doc.make_tax_gl_entries(gl_entries)
+		doc.make_internal_transfer_gl_entries(gl_entries)
+
+		doc.make_item_gl_entries(gl_entries)
+
+		disable_sdbnb_in_sr = frappe.get_cached_value("Company", doc.company, "disable_sdbnb_in_sr")
+
+		if not (doc.is_return and disable_sdbnb_in_sr):
+			doc.stock_delivered_but_not_billed_gl_entries(gl_entries)
+
+		doc.make_precision_loss_gl_entry(gl_entries)
+		doc.make_discount_gl_entries(gl_entries)
+
+		gl_entries = make_regional_gl_entries(gl_entries, doc)
+
+		# merge gl entries before adding pos entries
+		gl_entries = merge_similar_entries(gl_entries)
+
+		doc.make_loyalty_point_redemption_gle(gl_entries)
+		doc.make_pos_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)
+		return gl_entries

erpnext/accounts/services/base_gl_composer.py

@@ -0,0 +1,19 @@
+# Copyright (c) 2015, Frappe Technologies Pvt. Ltd. and Contributors
+# License: GNU General Public License v3. See license.txt
+
+"""Base class for per-document GL entry composers.
+
+A composer assembles the list of GL entry dicts for a single voucher. Unlike
+the posting sink (``general_ledger.make_gl_entries``) and the stateless
+validators (``gl_validator``), composing is stateful and per-document, so it is
+modelled as a class holding the document being composed. Subclasses implement
+``compose`` to return the voucher-specific list of GL entries.
+"""
+
+
+class BaseGLComposer:
+	def __init__(self, doc):
+		self.doc = doc
+
+	def compose(self):
+		raise NotImplementedError