N4/E1: T3 housing payoffs + a home-rest "well-rested" buff

Turn the dead top housing tiers into something worth buying. All three
land without a schema bump, and TestCombatCharacterization stays
byte-identical (the balance corpus never sets the new fields).

T3 trophy room: treasure cap 3->4 at HouseTier>=3 (maxTreasuresForTier).
Enforced at the save gate only -- HouseTier is never written downward,
so a held 4th treasure below tier 3 is unreachable and a load-time cap in
computeAdvBonuses would just add a query for an impossible state. The
all-irreplaceable manual discard prompt now lists the 4th slot too.

T3 workshop: +5% craft success at HouseTier>=3, lifting the cap 0.95->0.98
so a maxed forager still gains. craftingSuccessRate takes a workshopBonus,
threaded through autoCraftConsumables and renderRecipesKnown.

Well-rested buff (replaces the plan's T4 "inn-quality rest", a verified
no-op -- home rest already equals inn rest). Home-only (the inn and a
tier-1 shack grant nothing), starts at T2 and grows through T4, expires at
the next long rest:
  - Temp HP cushion (8/12/16% of MaxHP). TempHP was a dormant field; wired
    into applyDnDHPScaling as MaxHP headroom -- additive and golden-safe.
  - Bonus spell slots (+1/+2/+3 at the caster's highest slot level), the
    real reward, lifting casters who trail on spell-pool richness.
    applyLongRestSpellSlots resets the pool to base then folds in the
    bonus; expiry is stateless (next rest's reset drops it).

Magnitudes are tunable defaults; revisit against the post-parties
re-baseline.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_treasure_test.go

@@ -79,3 +79,13 @@ func TestAdvLocForZone(t *testing.T) {
 		t.Errorf("name = %q, want the zone's display name", loc.Name)
 	}
 }
+
+// TestMaxTreasuresForTier pins the T3 "trophy room" 4th slot: the base cap is
+// 3, and only a tier-3+ home lifts it to 4.
+func TestMaxTreasuresForTier(t *testing.T) {
+	for tier, want := range map[int]int{0: 3, 1: 3, 2: 3, 3: 4, 4: 4} {
+		if got := maxTreasuresForTier(tier); got != want {
+			t.Errorf("maxTreasuresForTier(%d) = %d, want %d", tier, got, want)
+		}
+	}
+}