Fail gracefully when extensions are missing

This still needs tests

lib/Arsse.php

@@ -8,6 +8,15 @@ namespace JKingWeb\Arsse;
 
 class Arsse {
     public const VERSION = "0.9.2";
+    public const REQUIRED_EXTENSIONS = [
+        "intl",      // as this extension is required to prepare formatted messages, its absence will throw a distinct English-only exception
+        "dom",
+        "filter",
+        "json",      // part of the PHP core since version 8.0
+        "hash",      // part of the PHP core since version 7.4
+        "simplexml", // required by PicoFeed only
+        "iconv",     // required by PicoFeed only
+    ];
 
     /** @var Factory */
     public static $obj;
@@ -29,6 +38,7 @@ class Arsse {
         static::$user = static::$user ?? new User;
     }
 
+    /** Checks whether the specified extensions are loaded and throws an exception if any are not */
     public static function checkExtensions(string ...$ext): void {
         $missing = [];
         foreach ($ext as $e) {

lib/CLI.php

@@ -80,11 +80,15 @@ USAGE_TEXT;
             'help' => false,
         ]);
         try {
+            // ensure the require extensions are loaded
+            Arsse::checkExtensions(...Arsse::REQUIRED_EXTENSIONS);
+            // reconstitute multi-token commands (e.g. user add) into a single string
             $cmd = $this->command($args);
             if ($cmd && !in_array($cmd, ["", "conf save-defaults", "daemon"])) {
                 // only certain commands don't require configuration to be loaded; daemon loads configuration after forking (if applicable)
                 $this->loadConf();
             }
+            // run the requested command
             switch ($cmd) {
                 case "":
                     if ($args['--version']) {

lib/REST.php

@@ -80,10 +80,12 @@ class REST {
     }
 
     public function dispatch(ServerRequestInterface $req = null): ResponseInterface {
-        // create a request object if not provided
-        $req = $req ?? ServerRequestFactory::fromGlobals();
-        // find the API to handle
         try {
+            // ensure the require extensions are loaded
+            Arsse::checkExtensions(...Arsse::REQUIRED_EXTENSIONS);
+            // create a request object if not provided
+            $req = $req ?? ServerRequestFactory::fromGlobals();
+            // find the API to handle
             [$api, $target, $class] = $this->apiMatch($req->getRequestTarget(), $this->apis);
             // authenticate the request pre-emptively
             $req = $this->authenticateRequest($req);

lib/Service/Daemon.php

@@ -7,9 +7,15 @@ declare(strict_types=1);
 namespace JKingWeb\Arsse\Service;
 
 use JKingWeb\Arsse\AbstractException;
+use JKingWeb\Arsse\Arsse;
 
 class Daemon {
     protected const PID_PATTERN = '/^([1-9]\d{0,77})?$/D'; // no more than 78 digits (256-bit unsigned integer), starting with a digit other than zero
+    protected const REQUIRED_EXTENSIONS = ["posix", "pcntl"];
+
+    public function __construct() {
+        Arsse::checkExtensions(...self::REQUIRED_EXTENSIONS);
+    }
 
     /** Daemonizes the process via the traditional sysvinit double-fork procedure
      *