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/steps/30-core.php

@@ -0,0 +1,21 @@
+<?php
+if (!defined('ABSPATH')) exit;
+
+return new class extends WPH_Step {
+    public function id(): string { return 'core'; }
+    public function title(): string { return 'Step 3 — WordPress Core Update'; }
+    public function sub_items(): array {
+        return [
+            'Check current version against latest stable release',
+            'If an update is available, apply it',
+            'After update, load the site front-end and wp-admin and confirm both are functioning',
+            'Check the database upgrade prompt — if WordPress prompts to upgrade the database, run it',
+            'Note the version you updated from and to',
+        ];
+    }
+    public function watch_outs(): array {
+        return [
+            'White screen of death post-update, admin redirect loops, missing admin menu items — these usually indicate a theme or plugin conflict with the new core version.',
+        ];
+    }
+};