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/README-unraid.md

@@ -85,14 +85,19 @@ templates, and the WebGUI install path.
 ## ━━━ THE WEB UI — THREE LAYERS ━━━
 
 ```
-  pages/*.php      what you look at        ── requires ──►  include/
+  pages/*.php      what you look at        ── polls ─────►  api/
   api/*.php        what the page fetches   ── requires ──►  include/
-  include/*.php    the actual logic — vv_*() functions, shared by both
+  include/*.php    the actual logic — vv_*() functions
 ```
 
-The split matters: a page and its API endpoint call **the same** `vv_*()` builders, so a
-cached response and a live one cannot disagree in shape. Adding a metric means adding it in
-`include/` once, and both the page and the endpoint get it.
+**Most pages hold no logic at all.** Eight of the eleven are pure view — markup, CSS and a
+poll loop, with every value arriving as JSON from `api/`. Only `auth.php`, `monitor.php` and
+`scheduler.php` require an `include/` file directly, and then only to server-render their
+initial state; their mutations still go through endpoints.
+
+That is why the layer split holds: a page and its endpoint cannot disagree about what a value
+means, because the page does not compute it. Adding a metric means adding it in `include/`
+once, and the endpoint, the cache writer, and the page all pick it up together.
 
 | Layer | Files | Role |
 |-------|-------|------|