[ADD] tax_receipt_in_portal: publish tax receipts on the portal

Let donors view and download their own tax receipts from
``/my/tax-receipts``: a paginated list, a detail page with the receipt
information, its invoices and the native portal chatter, and a PDF
download rendered on demand through the configured ``pdf_form`` report.

The PDF rendering is delegated to ``pdf_form_report`` and nothing is
archived: a confirmed receipt is frozen by ``l10n_fr_tax_receipt``, so
rendering it again yields the same document.

Two safety nets compensate third-party behaviors:

- report resolution by ``report_uuid`` (system parameter
  ``tax_receipt_in_portal.report_uuid``) with strict validation of
  report type and target model;
- a blank-field guard which resolves every mapped PDF field value
  before rendering, because ``pdf_form_report`` swallows resolution
  errors and would otherwise silently serve a receipt with a blank
  amount.

Access is enforced twice: a portal record rule (own commercial partner,
confirmed/sent state only) and an explicit controller domain that also
covers internal users browsing ``/my``.

Tested with 17 tests (portal isolation, draft exposure, chatter data
access, blank-field guard, report misconfiguration, private API
contract of ``pdf_form_report``), including a mutation check proving
the security tests fail when the record rule is opened.

tax_receipt_in_portal/security/ir.model.access.csv

@@ -0,0 +1,2 @@
+id,name,model_id:id,group_id:id,perm_read,perm_write,perm_create,perm_unlink
+access_l10n_fr_tax_receipt_portal,l10n.fr.tax.receipt portal read-only,l10n_fr_tax_receipt.model_l10n_fr_tax_receipt,base.group_portal,1,0,0,0

tax_receipt_in_portal/security/ir_rule.xml

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<odoo>
+    <!--
+        A donor may only read the receipts issued to their own commercial
+        entity, and only once confirmed: a draft receipt carries no legal
+        number yet.
+
+        Odoo scopes portal invoices through `message_partner_ids`, i.e. on
+        followers. That is deliberately not used here: tax receipts created
+        by the yearly generation wizard have no follower, so such a rule
+        would always yield an empty list.
+    -->
+    <record id="tax_receipt_rule_portal" model="ir.rule">
+        <field name="name">Portal: own confirmed tax receipts</field>
+        <field
+            name="model_id"
+            ref="l10n_fr_tax_receipt.model_l10n_fr_tax_receipt"
+        />
+        <field name="domain_force">
+            [('partner_id', 'child_of', [user.commercial_partner_id.id]),
+             ('state', 'in', ['confirmed', 'sent'])]
+        </field>
+        <field name="groups" eval="[(4, ref('base.group_portal'))]" />
+        <field name="perm_read" eval="True" />
+        <field name="perm_write" eval="False" />
+        <field name="perm_create" eval="False" />
+        <field name="perm_unlink" eval="False" />
+    </record>
+</odoo>