Add Instructors admin page (create + per-capability access)

Completes the instructor-management half of #9: the studio admin can now
create instructor accounts and toggle each instructor's capabilities.

- InstructorController (manage_instructors): list instructors, create a
  us_instructor WP user (emailing a set-password link), and a per-instructor
  capability detail view.
- InstructorCapabilities: pure, unit-tested rules for which managed caps an
  admin may assign and how a submitted form maps to assignments. Managed caps
  are manage_offerings, manage_questions, view_own_payments, export_payments;
  manage_availability and view_own_lessons are core to every instructor.
- A studio admin can never grant a capability it does not itself hold: only
  held caps (checked via current_user_can, so an administrator's dynamic grant
  counts) are offered, and on creation any managed cap the admin lacks is
  denied on the new instructor so they never exceed their creator. The role
  grants the managed caps by default; the page layers per-user overrides.
- AdminMenu: register the Instructors page in the people section.
- Tests for the capability logic; docs/features/user-roles.md updated.

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

tests/Unit/Auth/InstructorCapabilitiesTest.php

@@ -0,0 +1,58 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Auth;
+
+use Unsupervised\Schedular\Auth\InstructorCapabilities;
+use Unsupervised\Schedular\Auth\RoleManager;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class InstructorCapabilitiesTest extends TestCase
+{
+    public function testGrantableReturnsOnlyManagedCapsTheAdminHolds(): void
+    {
+        $grantable = InstructorCapabilities::grantable([
+            RoleManager::CAP_MANAGE_OFFERINGS => true,
+            RoleManager::CAP_EXPORT_PAYMENTS  => true,
+            // A non-instructor cap the admin holds is irrelevant here.
+            RoleManager::CAP_MANAGE_POLICIES  => true,
+        ]);
+
+        self::assertContains(RoleManager::CAP_MANAGE_OFFERINGS, $grantable);
+        self::assertContains(RoleManager::CAP_EXPORT_PAYMENTS, $grantable);
+        self::assertNotContains(RoleManager::CAP_MANAGE_QUESTIONS, $grantable);
+        self::assertNotContains(RoleManager::CAP_MANAGE_POLICIES, $grantable);
+    }
+
+    public function testGrantableExcludesCapsTheAdminLacksOrHasDisabled(): void
+    {
+        self::assertSame([], InstructorCapabilities::grantable([
+            RoleManager::CAP_MANAGE_OFFERINGS => false,
+        ]));
+
+        self::assertSame([], InstructorCapabilities::grantable([]));
+    }
+
+    public function testResolveMapsEachGrantableCapToWhetherItWasSubmitted(): void
+    {
+        $resolved = InstructorCapabilities::resolve(
+            [RoleManager::CAP_MANAGE_OFFERINGS],
+            [RoleManager::CAP_MANAGE_OFFERINGS, RoleManager::CAP_EXPORT_PAYMENTS]
+        );
+
+        self::assertTrue($resolved[RoleManager::CAP_MANAGE_OFFERINGS]);
+        self::assertFalse($resolved[RoleManager::CAP_EXPORT_PAYMENTS]);
+    }
+
+    public function testResolveIgnoresSubmittedCapsOutsideTheGrantableSet(): void
+    {
+        // The form posts a cap the admin may not grant; it must be dropped, so a
+        // studio admin can never assign a capability it does not itself hold.
+        $resolved = InstructorCapabilities::resolve(
+            [RoleManager::CAP_MANAGE_QUESTIONS, RoleManager::CAP_MANAGE_POLICIES],
+            [RoleManager::CAP_MANAGE_OFFERINGS]
+        );
+
+        self::assertSame([RoleManager::CAP_MANAGE_OFFERINGS => false], $resolved);
+    }
+}