Phase 1 MVP: drop-in step registry + session + Markdown report

Plugin skeleton, drop-in step registry, option-backed session, single-page
checklist UI, downloadable Markdown report. Steps live as one file each
under includes/steps/ — adding/removing one is a single file change.
Step IDs are stable strings so renaming files preserves session data.

Architecture (hc-5ix.3): WPH_Steps singleton globs includes/steps/*.php,
natsort-orders by filename, requires each file (which returns a WPH_Step
instance), then applies a 'wph_steps' filter so installs can drop steps.

Session (hc-5ix.4): option-backed (per decision — plugin is installed
per-engagement, so DB-resident history would be lost on uninstall).
Single in-progress session per site; finished sessions render a report
that the user downloads/copies.

Recovery bootstrap (hc-5ix.1): detects whether wp-site-recovery is
installed + active, surfaces state on the start panel and in every
active session. Manual install for now; private update channel deferred
to hc-5ix.27.

Smoke-tested on testsite: registry discovery (13 steps in correct order),
start → update_step → progress count → finish → 8KB Markdown report →
discard cycle.

includes/class-wph-steps.php

@@ -0,0 +1,55 @@
+<?php
+if (!defined('ABSPATH')) exit;
+
+/**
+ * Step registry. Singleton.
+ *
+ * discover() globs a directory for *.php files in sort order and requires each.
+ * Each file MUST return a WPH_Step instance. The numeric filename prefix
+ * (00, 10, 20...) controls the visual order.
+ *
+ * After discovery, the 'wph_steps' filter lets installation-specific code
+ * remove/replace steps (return the array keyed by step id).
+ */
+final class WPH_Steps {
+
+    private static ?WPH_Steps $instance = null;
+
+    /** @var array<string,WPH_Step> */
+    private array $steps = [];
+
+    private bool $discovered = false;
+
+    public static function instance(): WPH_Steps {
+        return self::$instance ??= new self();
+    }
+
+    public function discover(string $dir): void {
+        if ($this->discovered) return;
+        $files = glob(rtrim($dir, '/') . '/*.php') ?: [];
+        // Natural sort so 100-* comes after 20-* (not after 10-*).
+        natsort($files);
+        $files = array_values($files);
+        foreach ($files as $file) {
+            $obj = require $file;
+            if ($obj instanceof WPH_Step) {
+                $this->steps[$obj->id()] = $obj;
+            }
+        }
+        /**
+         * Filter the loaded steps. Return an array keyed by step id.
+         * To drop a step on a particular install: unset($steps['backup']).
+         */
+        $this->steps = apply_filters('wph_steps', $this->steps);
+        $this->discovered = true;
+    }
+
+    /** @return array<string,WPH_Step> */
+    public function all(): array {
+        return $this->steps;
+    }
+
+    public function get(string $id): ?WPH_Step {
+        return $this->steps[$id] ?? null;
+    }
+}