Add HST/tax support and payment reporting with HST aggregation

Studio Settings gains a default HST rate; the rate is frozen onto each
payment at booking and computed against the pre-tax subtotal, with the
total billed as subtotal + tax. The rate is overridable per booking on
My Lessons while unpaid (recomputing the tax amount), comped
registrations are never taxed, and receipts break out subtotal/HST/total.

Builds the payments report (roadmap #8) from us_payments: a monthly
per-instructor view with subtotal, HST collected, and grand-total
aggregation, plus a nonce-protected CSV export via admin-post. Studio
admins see all instructors and can filter; instructors are scoped to
their own rows. The Payment Report menu is gated on export_payments so
instructors (who lack manage_billing) can reach it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Unit/Payment/ReceiptMailerTest.php

@@ -32,4 +32,23 @@ class ReceiptMailerTest extends TestCase
 
         self::assertTrue((new ReceiptMailer())->send($payment, $student));
     }
+
+    public function testReceiptBreaksOutHstWhenTaxed(): void
+    {
+        Functions\expect('wp_mail')
+            ->once()
+            ->with(
+                'a@b.test',
+                Mockery::type('string'),
+                Mockery::on(static fn (string $body): bool => str_contains($body, 'HST') && str_contains($body, '113.00'))
+            )
+            ->andReturn(true);
+
+        $student             = Mockery::mock(\WP_User::class);
+        $student->user_email = 'a@b.test';
+
+        $payment = new Payment(5, 3, Payment::REG_LESSON, 12, 100.00, taxRate: 13.0, taxAmount: 13.00, receiptNumber: 'USC-1');
+
+        self::assertTrue((new ReceiptMailer())->send($payment, $student));
+    }
 }