Document the api layer's operational models, and fix two headers that had gone stale against the code

Plugin/unraid/api/node_chat.php

@@ -4,12 +4,19 @@
 //   The Partnership tab's mesh chat: read a channel, post to it, forget a message on this
 //   machine, and mark a channel read.
 //
-// REQUEST
-//   GET                                     channels + this host's id + unread counts
-//   GET  ?channel=<id>                      that channel's messages
-//   POST action=send    channel=<id> text=… [color=#rrggbb] [font=mono|sans|serif]
-//   POST action=delete  channel=<id> id=<msgid>          local only
-//   POST action=read    channel=<id>                     mark seen up to now
+// OPERATIONAL MODEL
+//   The method is the routing. Anything that is not a POST is a read — channel list or one
+//   channel's messages — and every POST carries an action. That keeps the CSRF boundary and the
+//   read/write boundary on the same line, so a mutation cannot arrive un-covered by being
+//   spelled as a GET.
+//
+//   Reads are computed per request, not cached. Unread counts come from walking the last 200
+//   messages of each channel against this host's read mark; the card polls on a slow cycle and
+//   the store is a small append log, so a cache would add a staleness class for no gain.
+//
+//   Delivery is not part of the response's success. vv_nc_send() writes locally and spools for
+//   any partner that could not be reached, and the reply carries that spool depth as `queued`.
+//   A sleeping partner is a pending message, not a failed one.
 //
 // DESIGN PRINCIPLES
 //   Read marks are local and per channel. "Unread" is a fact about this operator at this
@@ -28,6 +35,22 @@
 //   locally and spooled for retry; saying "failed" over something that is stored and queued
 //   would be the wrong claim.
 //
+// REQUEST
+//   GET                                     channels + this host's id + unread counts
+//   GET  ?channel=<id>                      that channel's messages
+//   POST action=send    channel=<id> text=… [color=#rrggbb] [font=mono|sans|serif]
+//   POST action=delete  channel=<id> id=<msgid>          local only
+//   POST action=read    channel=<id>                     mark seen up to now
+//
+// RESPONSE
+//   {"ok":true,"me":<host>,"channels":[{…,"unread":N}],"hostnames":{id:name}}   channel list
+//   {"ok":true,"me":<host>,"channel":<id>,"messages":[…],"last_read":<ts>}      one channel
+//   {"ok":true,"msg":{…},"queued":N}                                           send; N = spooled
+//   {"ok":true|false}                                                          delete
+//   {"ok":true}                                                                read
+//   {"ok":false,"error":"Unknown channel"|"Nothing to send"|"Could not store message"
+//                      |"No message id"|"Unknown action"}
+//
 // DEPENDS ON
 //   include/node_chat.php
 // ═══════════════════════════════════════════════════════════════════════════════════════════════