Review follow-up K: a fourth event anchor for the grind loop

The three A6 presence anchors (expedition Night digest, !sell, arena
cashout) miss a whole playstyle: a player who only mines/forages/fishes
(now automatic, done by walking) and clears single-day dungeons, but
never sells, never enters the arena, and never runs a multi-day
expedition to a Night camp, would roll for zero mid-day events.

Anchor a fourth roll on a foreground single-day zone clear — the climax
DM that player is reading, and one they cannot spam (a clear costs a
full walk). advanceResult.zoneCleared is set only in the full-clear
branch (a mid-zone region clear continues the run and would fire per
region), and the roll lives in zoneCmdAdvance, the foreground path only:
autopilot goes through runAutopilotWalk and party members are refused
earlier by isPartyMember. The per-day slot guard still caps everyone at
one event/day, so the three prior anchors are unmoved.

advEventChanceZoneClear = 0.08 puts a zone-clear-only player at ~1
event/week, matching the fully-engaged player; it is the tuning knob.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_events.go

@@ -32,10 +32,23 @@ type advActiveEvent struct {
 //
 // The per-anchor chances below put a player who hits all three at ~1 event
 // per week; see TestAnchoredEventWeeklyRate.
+//
+// K — the three social/economic anchors miss the pure grind loop: a player who
+// only mines/forages/fishes (now automatic, done by walking) and clears
+// single-day dungeons, but never sells, never enters the arena, and never runs
+// a multi-day expedition to a Night camp, would roll for zero events. The
+// fourth anchor, ZoneClear, fires on a foreground single-day zone clear — the
+// climax DM that player *is* reading — so they get the same rough cadence.
+// Autopilot walks and mid-zone region clears deliberately do not fire it (see
+// zoneCmdAdvance / the full-clear branch of advanceOnceWithOpts). The per-day
+// slot still caps everyone at one event, so a player who happens to hit two
+// anchors in a day can't double up. This rate is the tuning knob for item K —
+// bump it if telemetry shows the grind-loop player still sees too few.
 const (
-	advEventChanceDigest = 0.08
-	advEventChanceSell   = 0.05
-	advEventChanceArena  = 0.05
+	advEventChanceDigest    = 0.08
+	advEventChanceSell      = 0.05
+	advEventChanceArena     = 0.05
+	advEventChanceZoneClear = 0.08
 )
 
 var (