adventure: give the Siege a war room instead of a Matrix-only rumour

The Siege is the one mechanic where the whole town works on a single
object, and it existed exclusively in Matrix — so anybody not in the room
at the time never knew it happened. A communal event nobody can see is a
communal event that fails.

gogobee now pushes the war room on the roster tick and Pete replaces its
copy, the same snapshot contract the board has and for the same reason:
the shared pool is state, not history, and a retried snapshot would be a
lie about how much HP is left.

/adventure/siege draws it. The load-bearing detail is one CSS line — the
health bar has a width transition, so a poll that lands a lower pool
slides the bar down instead of snapping it. That is the difference
between watching the town chip a boss down and reading a report about it.
Alongside: a countdown to the window's close, past sieges with the bar
each one ended on, and the muster split into who took today's bout and
who still has one going spare — the mechanic is one fight per person per
day, so that second column is a hit the town hasn't taken yet.

The opt-out rule here deliberately differs from the board's. The board
omits an opted-out player outright, because class + level + zone
re-identifies them. A Siege contributor is anonymised instead: their
damage is part of what the town did to the boss, and deleting it would
understate the shared effort and stop the totals adding up. They keep
their rank, lose their name, and carry no token — so there is no link
back to a page that names them. An opted-out player who never fought is
still omitted; there is nothing to account for.

siege_start / siege_win / siege_loss are wired on the gogobee side; the
templates for all three have been sitting unused in renderAdventure
since the section shipped.

internal/web/handlers.go

@@ -126,6 +126,12 @@ type channelPage struct {
 	Roster      []RosterView
 	RosterStale bool
 	ShowRoster  bool
+
+	// Siege is the war room, summarised into a strip at the top of the section.
+	// Same page-1-only rule as the roster and for the same reason. It renders
+	// even with nothing camped, because the link to the history is the other half
+	// of what makes a live Siege feel like it counts.
+	Siege SiegeView
 }
 
 type indexPage struct {
@@ -353,6 +359,7 @@ func (s *Server) handleChannel(w http.ResponseWriter, r *http.Request, ch Channe
 	if ch.Slug == "adventure" && page == 1 {
 		data.Roster, data.RosterStale, _ = s.roster()
 		data.ShowRoster = true
+		data.Siege = s.siege()
 	}
 	s.render(w, "channel", data)
 }