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.go

@@ -50,6 +50,26 @@ var advTreasureDropRates = map[int]float64{
 
 const advMaxTreasures = 3
 
+// houseTierTrophyRoom is the T3 "Comfortable" house that unlocks the trophy
+// room — a fourth treasure slot.
+const houseTierTrophyRoom = 3
+
+// maxTreasuresForTier returns how many treasures a player may hold. The base
+// cap is 3; a T3+ house adds the trophy-room slot for a 4th.
+//
+// The extra slot is a housing perk, but HouseTier is only ever written upward
+// (purchase paths; there is no foreclosure/downgrade that writes it back down),
+// so a player can only ever hold a 4th treasure while tier>=3. There is
+// therefore no reachable state where a held treasure must be revoked at
+// bonus-computation time, and the cap is enforced only at the save gate.
+func maxTreasuresForTier(houseTier int) int {
+	n := advMaxTreasures
+	if houseTier >= houseTierTrophyRoom {
+		n++
+	}
+	return n
+}
+
 // ── Treasure Definitions ─────────────────────────────────────────────────────
 
 var advAllTreasures = map[int][]AdvTreasureDef{