Combat: round half-up on HP wound persistence to fix 101→100 drift

Wound carry-over went through three integer truncations across the
legacy combat scale (123 max) and the dndChar scale (78 max). 101/123
persisted as int(64.04)=64, then restored as int(100.92)=100 — losing
0.92 HP every fight.

Switching both sides to math.Round means most HPs round-trip exactly;
the residual error is bounded by ±1 HP from the inherent scale mismatch
(1 dndChar HP ≈ 1.58 legacy HP, so some pairs collide on the smaller
scale). Acceptable; the prior always-truncate behavior was directionally
biased against the player.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/dnd_combat.go

@@ -2,6 +2,7 @@ package plugin
 
 import (
 	"log/slog"
+	"math"
 	"time"
 
 	"gogobee/internal/db"
@@ -429,7 +430,11 @@ func applyDnDHPScaling(stats *CombatStats, c *DnDCharacter) {
 	if pct < dndWoundFloor {
 		pct = dndWoundFloor
 	}
-	scaled := int(float64(stats.MaxHP) * pct)
+	// Round half-up. Naive int truncation loses up to 1 HP on every
+	// round-trip through the dndChar scale (which is coarser than the
+	// legacy combat scale): 101/123 → persist as 64/78 → restore as
+	// int(100.92) = 100, silently dropping a HP between fights.
+	scaled := int(math.Round(float64(stats.MaxHP) * pct))
 	if scaled < 1 {
 		scaled = 1
 	}
@@ -470,7 +475,10 @@ func persistDnDHPAfterCombat(userID id.UserID, legacyStartHP, legacyEndHP int) {
 		pct = 1
 	}
 
-	newHP := int(float64(c.HPMax) * pct)
+	// Round half-up to mirror applyDnDHPScaling — a fight that ends at
+	// 101/123 should round-trip cleanly through the dndChar (78-scale)
+	// store and come back as 101, not 100.
+	newHP := int(math.Round(float64(c.HPMax) * pct))
 	if newHP < 0 {
 		newHP = 0
 	}