feat(nexa_coa_setup): 8 fiscal positions + tax substitution maps

XML defines 8 positions with auto-detection by country/state:
- CA Ontario (default), CA Atlantic, CA Quebec, CA BC, CA Prairies/Territories
- Export US, Export International, Tax Exempt

post_init hook _configure_fiscal_position_tax_maps sets up bidirectional
tax routing (sale + purchase) from the default '5% GST' to the appropriate
provincial tax via Odoo 19's account.fiscal.position.tax_ids /
account.tax.original_tax_ids relation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nexa_coa_setup/data/04_account_fiscal_position.xml

@@ -1,5 +1,64 @@
 <?xml version="1.0" encoding="utf-8"?>
 <odoo>
     <data noupdate="0">
+
+        <!-- Tax substitutions on each fiscal position are configured by the
+             post_init_hook (_configure_fiscal_position_tax_maps) because the
+             source/destination tax IDs are resolved at runtime from the
+             curated active tax set. -->
+
+        <record id="fp_ca_ontario" model="account.fiscal.position">
+            <field name="name">CA — Ontario (Default)</field>
+            <field name="auto_apply" eval="True"/>
+            <field name="country_id" ref="base.ca"/>
+            <field name="state_ids" eval="[(6, 0, [ref('base.state_ca_on')])]"/>
+        </record>
+
+        <record id="fp_ca_atlantic" model="account.fiscal.position">
+            <field name="name">CA — Atlantic (HST 15%)</field>
+            <field name="auto_apply" eval="True"/>
+            <field name="country_id" ref="base.ca"/>
+            <field name="state_ids" eval="[(6, 0, [ref('base.state_ca_nb'), ref('base.state_ca_ns'), ref('base.state_ca_pe'), ref('base.state_ca_nl')])]"/>
+        </record>
+
+        <record id="fp_ca_quebec" model="account.fiscal.position">
+            <field name="name">CA — Quebec (GST + QST)</field>
+            <field name="auto_apply" eval="True"/>
+            <field name="country_id" ref="base.ca"/>
+            <field name="state_ids" eval="[(6, 0, [ref('base.state_ca_qc')])]"/>
+        </record>
+
+        <record id="fp_ca_bc" model="account.fiscal.position">
+            <field name="name">CA — British Columbia (GST 5%, PST per-product)</field>
+            <field name="auto_apply" eval="True"/>
+            <field name="country_id" ref="base.ca"/>
+            <field name="state_ids" eval="[(6, 0, [ref('base.state_ca_bc')])]"/>
+        </record>
+
+        <record id="fp_ca_prairies_territories" model="account.fiscal.position">
+            <field name="name">CA — Prairies / Territories (GST 5% only)</field>
+            <field name="auto_apply" eval="True"/>
+            <field name="country_id" ref="base.ca"/>
+            <field name="state_ids" eval="[(6, 0, [ref('base.state_ca_ab'), ref('base.state_ca_mb'), ref('base.state_ca_sk'), ref('base.state_ca_yt'), ref('base.state_ca_nt'), ref('base.state_ca_nu')])]"/>
+        </record>
+
+        <record id="fp_export_us" model="account.fiscal.position">
+            <field name="name">Export — United States (Zero-rated)</field>
+            <field name="auto_apply" eval="True"/>
+            <field name="country_id" ref="base.us"/>
+        </record>
+
+        <record id="fp_export_intl" model="account.fiscal.position">
+            <field name="name">Export — International (Zero-rated)</field>
+            <field name="auto_apply" eval="False"/>
+            <field name="note">Manually apply for non-CA / non-US customers. Auto-apply by country-group requires a custom rule.</field>
+        </record>
+
+        <record id="fp_tax_exempt" model="account.fiscal.position">
+            <field name="name">Tax Exempt (cert-holder)</field>
+            <field name="auto_apply" eval="False"/>
+            <field name="note">Apply manually to customers with a valid exemption certificate on file. Record certificate details in the partner notes.</field>
+        </record>
+
     </data>
 </odoo>

nexa_coa_setup/hooks.py

@@ -80,10 +80,89 @@ def post_init_hook(env):
     _archive_unused_l10n_ca_accounts(env)
     _rename_legacy_accounts(env)
     _archive_unused_taxes(env)
+    _configure_fiscal_position_tax_maps(env)
     _lock_fiscal_year_2025(env)
     _logger.info("nexa_coa_setup: post_init_hook complete")
 
 
+def _configure_fiscal_position_tax_maps(env):
+    """For each Nexa fiscal position, set up tax substitution from default
+    '5% GST' to the appropriate provincial tax.
+
+    Odoo 19 fiscal position model:
+      - account.fiscal.position has tax_ids (M2M) — destination taxes
+      - account.tax has original_tax_ids (M2M) — source taxes it replaces
+      - Effective map is computed on the fly from both sides.
+
+    Idempotent: writes are 'replace' style (6, 0, [ids]) so re-running cleans
+    up prior runs.
+    """
+    def find_tax(name, use):
+        return env["account.tax"].search(
+            [("name", "=", name), ("type_tax_use", "=", use), ("active", "=", True)],
+            limit=1,
+        )
+
+    GST_5_sale = find_tax("5% GST", "sale")
+    GST_5_purchase = find_tax("5% GST", "purchase")
+    HST_13_sale = find_tax("13% HST", "sale")
+    HST_13_purchase = find_tax("13% HST", "purchase")
+    HST_15_sale = find_tax("15% HST", "sale")
+    HST_15_purchase = find_tax("15% HST", "purchase")
+    QST_sale = find_tax("14.975% GST+QST", "sale")
+    QST_purchase = find_tax("14.975% GST+QST", "purchase")
+    ZERO_sale = find_tax("0% GST", "sale")
+    ZERO_purchase = find_tax("0% GST", "purchase")
+
+    # (fp_xmlid, [(source_tax, destination_tax), ...])
+    fp_map = [
+        ("nexa_coa_setup.fp_ca_ontario", [
+            (GST_5_sale, HST_13_sale),
+            (GST_5_purchase, HST_13_purchase),
+        ]),
+        ("nexa_coa_setup.fp_ca_atlantic", [
+            (GST_5_sale, HST_15_sale),
+            (GST_5_purchase, HST_15_purchase),
+        ]),
+        ("nexa_coa_setup.fp_ca_quebec", [
+            (GST_5_sale, QST_sale),
+            (GST_5_purchase, QST_purchase),
+        ]),
+        # CA-BC and CA-Prairies/Territories: default is already 5% GST, no substitution
+        ("nexa_coa_setup.fp_ca_bc", []),
+        ("nexa_coa_setup.fp_ca_prairies_territories", []),
+        ("nexa_coa_setup.fp_export_us", [
+            (GST_5_sale, ZERO_sale),
+        ]),
+        ("nexa_coa_setup.fp_export_intl", [
+            (GST_5_sale, ZERO_sale),
+        ]),
+        ("nexa_coa_setup.fp_tax_exempt", [
+            (GST_5_sale, ZERO_sale),
+        ]),
+    ]
+
+    configured = 0
+    for fp_xmlid, pairs in fp_map:
+        fp = env.ref(fp_xmlid, raise_if_not_found=False)
+        if not fp:
+            _logger.warning("nexa_coa_setup: fiscal position not found: %s", fp_xmlid)
+            continue
+        # Clear existing destination taxes on the FP
+        fp.tax_ids = [(5, 0, 0)]
+        # For each (src, dst) pair, add dst to FP's tax_ids and src to dst's
+        # original_tax_ids
+        for src, dst in pairs:
+            if not src or not dst or src == dst:
+                continue
+            fp.tax_ids = [(4, dst.id, 0)]
+            existing_sources = dst.original_tax_ids.ids
+            if src.id not in existing_sources:
+                dst.original_tax_ids = [(4, src.id, 0)]
+        configured += 1
+    _logger.info("nexa_coa_setup: configured tax maps on %d fiscal positions", configured)
+
+
 # Tax names to keep ACTIVE (covers GST/HST/QST/PST across provinces + zero-rated
 # export + exempt). Everything else gets archived if it has zero usage on
 # existing journal entries.