N6/C3 review cleanup: retire the combined-level reducer duplication

The W1 extraction added combinedAdvLevel but left twinBeeMaxTier and
distributeTwinBeeRewards inlining the identical dndLevel+3-skills expression,
so the reducer lived in three places. Rewire both twinbee sites to
combinedAdvLevel — pure refactor, byte-identical arithmetic.

Deliberately did NOT share tierForCombinedLevel's switch into twinbee: its
default arm is worldBossMinTier, a world-boss knob, and coupling twinbee's
tier floor to it would let a world-boss retune silently leak into TwinBee's
activity selection. The reducer is the safe shared piece; the tier thresholds
stay independent.

Suite green, combat golden byte-identical.

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

internal/plugin/adventure_twinbee.go

@@ -40,7 +40,7 @@ func twinBeeMaxTier() int {
 		if !c.Alive {
 			continue
 		}
-		combined := dndLevelForUser(c.UserID) + c.MiningSkill + c.ForagingSkill + c.FishingSkill
+		combined := combinedAdvLevel(&c)
 		if combined > bestLevel {
 			bestLevel = combined
 		}
@@ -283,7 +283,7 @@ func (p *AdventurePlugin) distributeTwinBeeRewards(result *TwinBeeResult) TwinBe
 		weight := 4 // minimum (level 1 in all 4 skills)
 		for _, c := range chars {
 			if c.UserID == uid {
-				weight = dndLevelForUser(c.UserID) + c.MiningSkill + c.ForagingSkill + c.FishingSkill
+				weight = combinedAdvLevel(&c)
 				if weight < 4 {
 					weight = 4
 				}