feat(bridge): protocol 6 — first-party clans, one board and five events

The `clans` board carries every clan the game will list, with its roster
and roles, plus the facts the website needs to trust it: enabled,
backend, supported, truncated, and whether the optional uMod Clans
plugin is loaded. It is re-sent on connect and on the 60-second cadence,
and a few seconds after any membership change. The five events are
clan.created, clan.disbanded, clan.member.added, clan.member.left and
clan.member.kicked.

What the game's assemblies dictated:
- The game has no enumerate-all call. The clan leaderboard is the only
  listing, and the game clamps it to 100 clans. A board at that ceiling
  says `truncated`, as does one that would not fit in 768 KiB (the
  sidecar drops lines over 1 MiB).
- The local backend is synchronous behind ValueTask. A task that has not
  completed means another backend (Nexus), which is refused with a reason
  rather than waited on from the main thread.
- A clan id is not an identity, so every frame carries createdMs.
- OnClanMemberAdded hands over a bare id before the game's cache
  refreshes, so the clan is read back, with a fallback to the id alone.
- uMod Clans raises OnClanDisbanded(string, List<ulong>), which has the
  same name and arity as the game's hook, so the bridge types it exactly.

A member's LastSeen is never sent: it is presence (PLAN.md §23).
overlay.toml moves to protocol 6 and drops Clans from requires_plugins
(D47): Teams come from the game's own clans.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

README.md

@@ -58,11 +58,15 @@ tarball and installs the sidecar alongside it.
 
 ### Prerequisites
 
-The bridge itself needs nothing but Oxide. The features that follow it read four third-party plugins
-an operator installs from uMod — `Clans`, `Kits`, `PopupNotifications` and `ZoneManager`. They are
+The bridge itself needs nothing but Oxide. The features that follow it read three third-party
+plugins an operator installs from uMod — `Kits`, `PopupNotifications` and `ZoneManager`. They are
 listed in `overlay.toml` so the installer's `doctor` can report a missing one by name rather than
 leaving the site quietly short of a feature.
 
+Clans on the website come from Rust's **own** clan system, not from the uMod `Clans` plugin, which
+is optional. The two share no data: a server running the plugin has two unrelated clan systems, and
+only the game's own clans become Teams on the site.
+
 ## Diagnosing it
 
 ```