Phase 0: expedition-difficulty sim harness spike

New file internal/plugin/expedition_balance.go — sibling to
dnd_class_balance.go, no dnd_ prefix per feedback_avoid_dnd_naming.
Plan doc: gogobee_expedition_difficulty.md.

Critical de-risk settled: clock-injection seam. The harness does not
run the wall-clock tickers (expeditionBriefingTicker @ 06:00,
expeditionRecapTicker @ 21:00, expedition_ambient @ 3h) and does not
inject a virtual time.Now. Instead, advanceExpeditionOneDay
reimplements the morning→day→night pipeline by calling the math-pure
helpers directly: applyDailyBurn, dailyThreatDrift,
resolveCombatInterrupt, resolveWanderingCheck, simulateCombatWithRNG.
No DB, no goroutines, no clock.

Phase 0 trade-offs (documented in the file header):
- Boss completion deferred; survive-N-days is the proxy.
- Per-region zones, loot/XP, pardon/Sovereign, babysit, temporal
  stack effects all skipped — Phase 1+ scope.
- Encounter cadence (harnessHarvestRollsPerDay=4) is a placeholder
  to be calibrated against live traces in Phase 1.

Tests:
- TestExpeditionBalance_Phase0_Spike — T2 Crypt Valdris × L5 Fighter,
  100 trials, asserts only degenerate sentinels (no 0%/100%, days>0,
  days<=cap). Runs in <3s.
- TestExpeditionBalance_Phase0_SeedSpread — confirms the RNG seam is
  actually wired by showing two distinct seeds produce different
  trials. Full byte-for-byte reproducibility under same seed is not
  asserted at Phase 0; surpriseRoundNick + pickWanderingMonster
  draw from package-global rand (same caveat as class balance).

Spike numbers (calibration baseline, NOT a target): 1% completion,
99% deaths, median 4 days, ~6 encounters. The headline death rate is
loud but expected — Phase 1's full matrix will diagnose whether it's
HP-carryover punishment, bestiary-vs-abstract-monster delta, or kit
ladder mismatch. Phase 2 is where the centerline gets tuned.

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

internal/plugin/expedition_balance_test.go

@@ -0,0 +1,93 @@
+package plugin
+
+import (
+	"testing"
+)
+
+// TestExpeditionBalance_Phase0_Spike is the sanity check the
+// expedition-difficulty plan doc's Phase 0 calls for: one cell, 100
+// trials, sensible numbers, fully deterministic from the seed.
+//
+// The cell — T2 zone (Crypt Valdris) × L5 Fighter — was chosen
+// because it's the smallest cell that actually exercises every part
+// of the seam: tier scaling raises monster AC/atk, supply burn lands
+// at 1.5×, the player is built off the class-balance loadout ladder
+// at the T2/+1 step, and the night phase has roster entries to draw
+// from. If the seam works here, Phase 1's full-matrix expansion is
+// mostly typing.
+//
+// Pass criteria are intentionally loose — we are not asserting a
+// target band yet (Phase 2 does that), only that nothing is wired up
+// so badly the result is degenerate (0%/100%/NaN/zero days). Anything
+// in (0..100%) completion at this cell is a green Phase 0.
+func TestExpeditionBalance_Phase0_Spike(t *testing.T) {
+	const trials = 100
+	profile := expeditionBalanceProfile{
+		ZoneID: ZoneCryptValdris,
+		Class:  ClassFighter,
+		Level:  5,
+		Supplies: makeSupplies(ZoneTierApprentice, SupplyPurchase{
+			StandardPacks: 3,
+			DeluxePacks:   0,
+		}),
+		CampType: CampTypeStandard,
+	}
+
+	res := runExpeditionBalanceCell(profile, trials, 0xC0FFEE)
+
+	t.Logf("cell: %s L%d %s — completions=%d/%d (%.1f%%), deaths=%d, starved=%d, "+
+		"median_days=%d, median_threat=%d, avg_encounters=%.1f, avg_hp_remaining=%.1f%%",
+		profile.ZoneID, profile.Level, profile.Class,
+		res.Completions, res.Trials, res.CompletionRate()*100,
+		res.Deaths, res.StarvedOuts,
+		res.MedianDays, res.MedianThreatEnd,
+		res.AvgEncounters, res.AvgHPRemainingPct*100,
+	)
+
+	// Degenerate-outcome gates. These are harness-broken sentinels,
+	// not difficulty assertions — Phase 2 layers the real band on
+	// top of this same test.
+	if res.Trials != trials {
+		t.Fatalf("trial count mismatch: got %d, want %d", res.Trials, trials)
+	}
+	if res.Completions == 0 {
+		t.Errorf("zero completions in %d trials at T2/L5 Fighter — the spike cell should not be unwinnable", trials)
+	}
+	if res.Completions == trials {
+		t.Errorf("100%% completions in %d trials at T2/L5 Fighter — interrupt rolls / night checks not pressuring the run", trials)
+	}
+	if res.MedianDays == 0 {
+		t.Fatalf("median days == 0; day loop never advanced")
+	}
+	if res.MedianDays > harnessMaxDays {
+		t.Fatalf("median days %d > cap %d; termination wiring broken", res.MedianDays, harnessMaxDays)
+	}
+}
+
+// TestExpeditionBalance_Phase0_SeedSpread confirms the RNG seam is
+// actually wired — different seeds produce different trial outcomes
+// across a small sample. Full byte-for-byte reproducibility under
+// the same seed is *not* asserted at Phase 0: a couple of production
+// helpers we lean on (surpriseRoundNick, pickWanderingMonster) draw
+// from the package-global math/rand/v2, same as the class-balance
+// harness. Phase 1 lifts those to seeded variants if matrix
+// reproducibility becomes a real requirement; until then, "seeds
+// differentiate" is the contract we can honestly hold.
+func TestExpeditionBalance_Phase0_SeedSpread(t *testing.T) {
+	profile := expeditionBalanceProfile{
+		ZoneID: ZoneCryptValdris,
+		Class:  ClassFighter,
+		Level:  5,
+		Supplies: makeSupplies(ZoneTierApprentice, SupplyPurchase{
+			StandardPacks: 3,
+		}),
+		CampType: CampTypeStandard,
+	}
+	const seedA uint64 = 0xDEADBEEF
+	const seedB uint64 = 0xFEEDFACE
+	a := runExpeditionBalanceTrial(profile, seedA)
+	b := runExpeditionBalanceTrial(profile, seedB)
+	if a == b {
+		t.Fatalf("two distinct seeds produced byte-identical trials — RNG seam may not be wired:\n a = %+v\n b = %+v", a, b)
+	}
+}