Add structured headers to the PHP pages layer

Documents each tab's purpose, what it renders, and which endpoints it polls.
Pages that start with markup get the header in a <?php ?> block so it never
reaches the browser.

Also corrects the layer diagram in README-unraid.md: eight of eleven pages hold
no PHP logic and poll api/ for everything — only auth, monitor and scheduler
require an include/ file directly.

Plugin/unraid/pages/setup.php

@@ -1,4 +1,37 @@
 <?php
+// ═══════════════════════════════════════════════════════════════════════════════════════════════
+// PURPOSE
+//   Setup tab. First-run wizard — walks a fresh install through host identity, credentials,
+//   API key creation, storage mode, and the readiness checklist.
+//
+// DESIGN PRINCIPLES
+//   The checklist is derived, never stored. api/checklist.php re-evaluates real state on each
+//   load, so a step cannot be marked complete while the thing it checks is actually missing.
+//
+//   Setup state is pushed to partners. Once a node knows who it is, that identity is shared
+//   through vv_push_setup_state() rather than typed twice.
+//
+//   Detection over interrogation. Where a value can be read from a running service, setup
+//   reads it instead of asking — the same principle conf_populate.sh follows.
+//
+// OPERATIONAL SAFEGUARDS
+//   Existing values are not overwritten by detection. A field already filled in stays as it
+//   is; the wizard fills gaps rather than resetting a working install.
+//
+//   Ambiguous detection is refused rather than guessed — a container prefix matching two
+//   containers is reported for manual resolution, never picked arbitrarily.
+//
+//   Credentials entered here are written to host*.conf, which is gitignored. They are never
+//   committed and never leave the node except through the explicit partner push.
+//
+// RENDERS
+//   Step-by-step wizard, readiness checklist, credential entry, API key creation
+//
+// DEPENDS ON
+//   api/setup.php           wizard state and writes
+//   api/checklist.php       live readiness evaluation
+//   api/create_api_key.php  unraid-api key provisioning
+//   api/storage.php         storage-mode selection
 // First-run setup wizard — uniform flow for all hosts.
 // Step 1: auto-detect environment + server identity form.
 // Step 2: auto-populate + guide + checklist.