J3 D8-review: surface sim subprocess errors + SW upcast + doorway msg

- expedition-sim matrix worker now captures child stderr and dumps
  runErr/stderr/stdout-snippet on failure so halted rows have a cause.
- simPickSpiritualWeapon walks slots 2..5 and upcasts when L2 is spent
  instead of silently skipping the spell on high-level clerics.
- advanceOnceWithOpts !inlineBossCombat branch now emits the same
  "Room X/Y — Boss/Elite. Type !fight to engage." line as foreground.

cmd/expedition-sim/main.go

@@ -13,6 +13,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"flag"
 	"fmt"
@@ -191,14 +192,33 @@ func matrixWorker(exe string, in <-chan matrixJob, out chan<- *plugin.SimResult,
 			args = append(args, "-trace")
 		}
 		cmd := exec.Command(exe, args...)
+		var stderrBuf bytes.Buffer
+		cmd.Stderr = &stderrBuf
 		stdout, runErr := cmd.Output()
 		var res plugin.SimResult
-		if jerr := json.Unmarshal(stdout, &res); jerr != nil {
+		jerr := json.Unmarshal(stdout, &res)
+		if jerr != nil {
 			res = plugin.SimResult{UserID: uid, Class: j.class, Level: j.level, Zone: j.zone, Outcome: "halted"}
 		}
 		if runErr != nil && res.Outcome == "" {
 			res.Outcome = "halted"
 		}
+		// Surface subprocess failures: parse error with non-empty stdout
+		// (corrupted JSON) and non-zero exits both get a stderr dump so the
+		// user sees the underlying cause instead of just a halted row.
+		if jerr != nil || runErr != nil {
+			fmt.Fprintf(os.Stderr, "sim cell %s halted: runErr=%v jerr=%v\n", uid, runErr, jerr)
+			if stderrBuf.Len() > 0 {
+				fmt.Fprintf(os.Stderr, "  child stderr:\n%s\n", stderrBuf.String())
+			}
+			if jerr != nil && len(stdout) > 0 {
+				snip := stdout
+				if len(snip) > 200 {
+					snip = snip[:200]
+				}
+				fmt.Fprintf(os.Stderr, "  child stdout (first 200B): %q\n", snip)
+			}
+		}
 		if !includeLog {
 			res.Log = nil
 		}

internal/plugin/dnd_zone_cmd.go

@@ -572,11 +572,17 @@ func (p *AdventurePlugin) advanceOnceWithOpts(ctx MessageContext, compact, inlin
 				// autoResolveCombat / simPickCombatAction). Surface the
 				// doorway like the foreground path does, after the safety
 				// gate has had a chance to defer the engagement.
+				kind := "Elite"
 				r := stopElite
 				if prev == RoomBoss {
+					kind = "Boss"
 					r = stopBoss
 				}
-				return advanceResult{reason: r}, nil
+				return advanceResult{
+					final: fmt.Sprintf("**Room %d/%d — %s.** Type `!fight` to engage.",
+						prevIdx+1, run.TotalRooms, kind),
+					reason: r,
+				}, nil
 			}
 			// Compact-mode elite/boss auto-resolve. resolveCombatRoom
 			// selects monster + label by run.CurrentRoomType().

internal/plugin/expedition_sim.go

@@ -894,13 +894,18 @@ func simMartialFirstClass(class DnDClass) bool {
 	return false
 }
 
-// simPickSpiritualWeapon returns "spiritual_weapon" when a cleric should
+// simPickSpiritualWeapon returns a !cast argument for Spiritual Weapon
-// open the fight with it: the buff is not already active on the session,
+// when a cleric should open the fight with it: the buff is not already
-// the spell is prepared, and an L2 slot is available. Returns "" otherwise.
+// active on the session, the spell is prepared, and some slot ≥ L2 is
-// The buff path in combat_cmd.go folds into BuffSpiritProc/Dmg via
+// available. Returns "" otherwise. The buff path in combat_cmd.go folds
-// applyBuffDelta, which the turn engine fires each round via
+// into BuffSpiritProc/Dmg via applyBuffDelta, which the turn engine fires
-// spiritWeaponStrike — so one cast is worth more than a single L2 damage
+// each round via spiritWeaponStrike — so one cast is worth more than a
-// spell across a multi-round fight.
+// single L2 damage spell across a multi-round fight.
+//
+// Slot pick: lowest available slot ≥ 2. Upcasting is +1d8 per 2 slots
+// above 2nd, so spending a precious L5 to add a single d8 to the proc is
+// not worth burning the bigger slot's damage potential elsewhere; sim
+// behaves like a competent player and saves the high slot.
 func simPickSpiritualWeapon(c *DnDCharacter, uid id.UserID, sess *CombatSession) string {
 	if c == nil || c.Class != ClassCleric || sess == nil {
 		return ""
@@ -923,11 +928,18 @@ func simPickSpiritualWeapon(c *DnDCharacter, uid id.UserID, sess *CombatSession)
 		return ""
 	}
 	slots, _ := getSpellSlots(uid)
-	pair, ok := slots[2]
+	const simMaxSlot = 5
-	if !ok || pair[0]-pair[1] <= 0 {
+	for sl := 2; sl <= simMaxSlot; sl++ {
-		return ""
+		pair, ok := slots[sl]
+		if !ok || pair[0]-pair[1] <= 0 {
+			continue
+		}
+		if sl == 2 {
+			return "spiritual_weapon"
+		}
+		return fmt.Sprintf("spiritual_weapon --upcast %d", sl)
 	}
-	return "spiritual_weapon"
+	return ""
 }
 
 // simPickSpell returns the spell argument a competent player would pass