refactor: rename lib/ to core/ for clearer core-module separation

Rename the top-level lib/ directory to core/ so the project root
immediately communicates which code is core platform and which lives
in modules/. PHP namespaces (MintyPHP\*) are unchanged — only the
Composer PSR-4 path mapping moves from lib/ to core/.

Module-internal lib/ directories (modules/*/lib/) are untouched.

Updated across the full stack:
- composer.json PSR-4 mapping
- bootstrap entry points (web/index.php, bin/*, tests/bootstrap.php)
- tooling configs (phpstan.neon, phpunit.xml, php-cs-fixer)
- 26 architecture contract tests
- enforcement-policy, guard-catalog, quality-gates
- all documentation (CLAUDE.md, README, 25 docs/, .agents/skills/)
- bin/qa-extended.sh search paths
- .claude/settings.local.json permission paths

Workflow: .agents/runs/CORE-LIB-RENAME-001/ (Analyst → Planner →
Executor → Code Review (4 findings fixed) → Security Review →
Acceptance Test → Finalizer)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/Http/SessionStore.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace MintyPHP\Http;
+
+class SessionStore implements SessionStoreInterface
+{
+    public function get(string $key, mixed $default = null): mixed
+    {
+        $session = $this->all();
+        if (!array_key_exists($key, $session)) {
+            return $default;
+        }
+
+        return $session[$key];
+    }
+
+    public function set(string $key, mixed $value): void
+    {
+        if (!is_array($_SESSION ?? null)) {
+            $_SESSION = [];
+        }
+
+        $_SESSION[$key] = $value;
+    }
+
+    public function has(string $key): bool
+    {
+        $session = $this->all();
+        return array_key_exists($key, $session);
+    }
+
+    public function remove(string $key): void
+    {
+        if (!is_array($_SESSION ?? null)) {
+            $_SESSION = [];
+        }
+
+        unset($_SESSION[$key]);
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    public function all(): array
+    {
+        /** @var array<string, mixed> $session */
+        $session = $_SESSION;
+        return $session;
+    }
+}