Phase 2a (lever): expedition retreat semantics split

Phase 2 diagnostics named the InterruptElite bracket as the likely
first lever; tier-walking the lethality probe at the matrix cadence
told a different story.  Phase 1's uniform-0% baseline isn't an
elite-bracket calibration issue at all — every tier reads 0% because
the engine's TimedOut contract was being ignored by every expedition
caller.

combat_engine.go:451 says: "Timeout = retreat, not lethal blow.
Caller treats a timeout loss as 'fight ended, no character death'".
But runHarvestInterrupt / tryPatrolEncounter / resolveCombatRoom all
called abandonZoneRun + retireAllRegionRuns on any !PlayerWon —
ending the expedition outright on a retreat.  The retreat flavor line
("X outlasts you. You retreat from the expedition, wounded but alive")
was already in the code, just stapled to an actual run-abort.

Splits the policy by caller:

  • runHarvestInterrupt   — autopilot daytime interrupt.  TimedOut →
    retreat: threat +5, HP carries over, run continues, harvest slot
    forfeit (no kill / loot).  HP<=0 still ends the run + marks dead.
  • tryPatrolEncounter   — !advance pre-room patrol roll.  Same
    retreat policy: patrols don't gate progress, so retreating from
    one and walking into the next room is the right shape.
  • resolveCombatRoom    — !advance room/elite combat.  Unchanged —
    this path gates room progression; a retreat has nowhere to go, so
    any loss still ends the run.  (Manual zone runs were always
    intended to end here.)

Harness mirrored: daytime interrupt timeout → carry HP + threat bump
+ continue day; night-encounter loss → terminate (mirrors
resolveCombatRoom, since live night encounters defer to !advance).

retreatThreatBump = 5 is the per-retreat threat penalty.  Low enough
not to compound brutally with chained retreats, high enough that 3–4
retreats noticeably walks the threat clock toward Stirring.  Easy to
dial in Phase 3 if zones go off-band.

Phase 1 matrix after the change still reads 0% completion at every
cell — but the encounter counts and survival shape are dramatically
different (T4 underdark 3.6→7.5 encs; T3 underforge trial saw 18
encounters across 10 days where the pre-change run died on day 2/3).
Adds TestExpeditionBalance_Phase2_TierLethality, a tier-walking
companion to the T1/rolls=1 probe, that traces every fight at the
matrix cadence across one zone per tier — the actual Phase 2b lever
work picks from this data, not the old T1-only probe.

The remaining 0% is now legibly driven by tier-disproportionate elite
rosters (Hobgoblin Warchief at T1, Green Hag at T2, Roper/Helmed
Horror higher up) that one-shot or two-shot tier-appropriate
fighters.  Phase 2b's lever shortlist:

  1. Roster gate / SpawnWeight tuning to dilute over-tier elites.
  2. Surprise-nick floor reduction on chained interrupts (carryover
     HP + nick is the death-spiral fingerprint at T1 specifically).
  3. Per-day cadence reduction if 1+2 don't carry T1 to band.

Pre-existing test failures (TestAdv2Scenario_ZoneRunGoblinWarrens,
TestMageSpellbookLineInRender) verified to fail identically on HEAD;
no new test regressions.

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

internal/plugin/expedition_balance_test.go

@@ -428,6 +428,73 @@ func TestExpeditionBalance_Phase2_LethalityProbe(t *testing.T) {
 	}
 }
 
+// TestExpeditionBalance_Phase2_TierLethality is the tier-walking
+// companion to the T1-only lethality probe. Phase 1's matrix shows
+// uniform 0% across every tier at the default rolls=4 cadence, which
+// the T1/rolls=1 probe alone can't explain (its bimodal-warchief
+// finding wouldn't show at T5 vs an L17 fighter). This probe traces
+// every fight at the matrix cadence across one zone per tier so we
+// can read whether the deaths are elite-driven, standard-fight
+// chained, or something the harness combat fold itself is doing.
+//
+// Diagnostic-only — no assertions.
+func TestExpeditionBalance_Phase2_TierLethality(t *testing.T) {
+	if testing.Short() {
+		t.Skip("phase 2 tier-lethality probe writes verbose log; -short skips it")
+	}
+
+	const trialsPerTier = 3
+	const baseSeed uint64 = 0x7E11A1
+	cells := []struct {
+		zone ZoneID
+		tier ZoneTier
+	}{
+		{ZoneGoblinWarrens, ZoneTierBeginner},
+		{ZoneForestShadows, ZoneTierApprentice},
+		{ZoneUnderforge, ZoneTierJourneyman},
+		{ZoneUnderdark, ZoneTierVeteran},
+		{ZoneDragonsLair, ZoneTierLegendary},
+	}
+	for _, cell := range cells {
+		level := phase1TierCenterline[cell.tier]
+		profile := expeditionBalanceProfile{
+			ZoneID:             cell.zone,
+			Class:              ClassFighter,
+			Level:              level,
+			Supplies:           makeSupplies(cell.tier, SupplyPurchase{StandardPacks: 3}),
+			CampType:           CampTypeStandard,
+			HarvestRollsPerDay: 4, // matrix default, not the sparse probe
+		}
+		t.Logf("═══ %s T%d L%d Fighter rolls=4 ═══", cell.zone, cell.tier, level)
+		for trial := 0; trial < trialsPerTier; trial++ {
+			exp := newHarnessExpedition(profile)
+			char := buildHarnessCharacter(classBalanceProfile{
+				Class: profile.Class,
+				Level: profile.Level,
+			})
+			t.Logf("─── %s trial %d: hp_max=%d ───", cell.zone, trial, char.HPMax)
+			h := &expeditionHarness{
+				exp:         exp,
+				char:        char,
+				rng:         newHarnessRNG(baseSeed + uint64(trial) + uint64(cell.tier)*101),
+				rollsPerDay: profile.HarvestRollsPerDay,
+				traceFight: func(line string) {
+					t.Logf("  %s", line)
+				},
+			}
+			for {
+				res := h.advanceExpeditionOneDay()
+				if res.EndedReason != "" {
+					t.Logf("END %s trial %d: reason=%s days=%d threat=%d encs=%d hp_left_pct=%.1f%%",
+						cell.zone, trial, res.EndedReason, res.DaysElapsed, res.ThreatAtEnd,
+						res.CombatEncounters, res.HPRemainingPct*100)
+					break
+				}
+			}
+		}
+	}
+}
+
 // joinZones is a tiny helper kept local to the test file so the
 // per-tier log line reads in one logical chunk without pulling in
 // strings.Join's import for production code.