Phase 4a (diag): per-zone outlier attribution

The Phase 3-B sweep wrung out the global lever surface and named four
zones that read ~0% completion under every (elite-threshold, drift,
nick-floor, supply-burn) combo: crypt_valdris T1, forest_shadows T2,
manor_blackspire T3, abyss_portal T5. Each has a healthy sibling at
the same tier (goblin_warrens 44%, sunken_temple 13.5%, underforge
2.5%, dragons_lair 57.5%) so the gap is per-zone, not tier-wide.

Adds a structured per-fight trace hook (traceFightStruct + the
harnessFightTrace struct that mirrors the existing string trace) so
diagnostics can aggregate without parsing the formatted log line.
Mirror-format with traceFight; if a field is added, update both
paths.

TestExpeditionBalance_Phase4A_OutlierDiagnostic walks the four
outlier-vs-sibling pairs at 200 trials each on the Phase 3-A/3-B best
cell (e=23 d=1 burn=50) and reports per-monster appearances /
win-rate / avg HP loss / kill attribution + day-of-end histogram +
elite-vs-standard fight mix.

Findings:
  - crypt_valdris: dual-killer elite pool (Wight 99 kills, Flameskull
    68). Phase 2c left this zone untouched ("already dual-elite") but
    both elites are over-tier for T1.
  - forest_shadows: standard pool too lethal — Displacer Beast (53
    kills, 38% win standard) + Bandit Captain (48 kills, 57% win).
  - manor_blackspire: Wraith on the standard slot is dragging the
    floor (85 kills, 45hp loss per win). Vampire Spawn + Revenant
    elite pair is also rough.
  - abyss_portal: Nalfeshnee mis-classified standard (86 kills, 2.8%
    win at L17). Vrock at 79% win is borderline.

Phase 4-B applies per-zone roster tweaks (IsElite re-flag, drop a
deadly entry, soften a SpawnWeight) — no monster stat-block changes.
T3 may need a follow-up tier-wide pass since the sibling underforge
also sits at 2.5%; out of scope here.

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

internal/plugin/expedition_balance.go

@@ -504,6 +504,21 @@ func (h *expeditionHarness) runHarnessFight(zone ZoneDefinition, elite bool) Com
 			monster.Name, h.char.HPMax, nick, hpBeforeFight, result.PlayerEndHP,
 			enemy.Stats.AC, enemy.Stats.AttackBonus, outcome))
 	}
+	if h.traceFightStruct != nil {
+		h.traceFightStruct(harnessFightTrace{
+			Day:         h.exp.CurrentDay,
+			Tier:        int(zone.Tier),
+			Elite:       elite,
+			MonsterName: monster.Name,
+			HPMax:       h.char.HPMax,
+			Nick:        nick,
+			HPPre:       hpBeforeFight,
+			HPPost:      result.PlayerEndHP,
+			EnemyAC:     enemy.Stats.AC,
+			EnemyAtk:    enemy.Stats.AttackBonus,
+			Won:         result.PlayerWon,
+		})
+	}
 	return result
 }
 
@@ -608,6 +623,30 @@ type expeditionHarness struct {
 	// monster, or the combat fold itself is driving deaths. Nil in
 	// production runs — has zero cost when unused.
 	traceFight func(line string)
+	// traceFightStruct, if non-nil, fires alongside traceFight with the
+	// same fight's data as a parsed struct. Lets aggregate diagnostics
+	// (e.g. Phase 4-A's per-monster attribution) skip the fragile parse
+	// of the formatted line. Retreat events are not surfaced here — the
+	// retreat trace is one-line only via traceFight. Nil in production.
+	traceFightStruct func(harnessFightTrace)
+}
+
+// harnessFightTrace is the structured per-fight record exposed via
+// traceFightStruct. Mirrors the fields the formatted traceFight line
+// already emits; if you add a field, update both paths so the human
+// log and the structured aggregate stay in sync.
+type harnessFightTrace struct {
+	Day         int
+	Tier        int
+	Elite       bool
+	MonsterName string
+	HPMax       int
+	Nick        int
+	HPPre       int
+	HPPost      int
+	EnemyAC     int
+	EnemyAtk    int
+	Won         bool
 }
 
 // harnessRNG is a thin wrapper around math/rand/v2 so the d20 helper