adventure: T6 postgame Layer-2 seam + Aurvandryx's Greed Tax (P8)

Add the pre-combat boss-hook seam for Tier-6 postgame bosses and the first
bespoke mechanic on it.

Seam: applyBossRunModifiers(bossID, enemy, run) in postgame_boss_hooks.go —
a pure, idempotent, bestiary-ID-dispatched hook that folds run-state-derived
adjustments into the freshly-built boss Combatant. No-op for every non-hooked
enemy or nil run. The turn engine (which resolves both prod bosses and the
sim) rebuilds the enemy every round via partyCombatantsForSession, so the hook
must be a pure function of run state — fine at a terminal boss room, where the
route is frozen. Wired at both enemy-finalization points: the per-round rebuild
and buildFightSeats' initial HP persist (threaded a run param; the caller
already had it).

Greed Tax (boss_aurvandryx / first_hoard): her Attack rises with the richness
of the route walked to reach her — the summed excess LootBias (>1.0) over the
run's visited nodes. Note run.LootCollected is the wrong signal (BossOnly
signature manifest, empty at the boss); the gilded veins on the graph are.
Attack += min(2.0*richness, 12). Same-binary A/B sweep (n=150, L20 party +
Pete + pets): taxless 66.7% -> taxed 48.0%, landing first_hoard mid-band.

Claude-Session: https://claude.ai/code/session_0156WqjgsbmSY2U8eQ3Kkb1s

internal/plugin/postgame_boss_hooks.go

@@ -0,0 +1,98 @@
+package plugin
+
+// Tier-6 postgame "Layer-2" boss mechanics (Phase P8).
+//
+// Layer 1 (shipped) is everything the stock engine expresses: a single
+// MonsterAbility rider plus HP/AC/Attack/PhaseTwoAt on the stat block. Layer 2
+// is the bespoke, per-boss stuff the plan promised — mechanics that read the
+// *run* the player took to reach the boss, not just the fight in front of them.
+//
+// This file holds the PRE-COMBAT half of that seam: adjustments derived once
+// from run state and folded into the boss's live Combatant before the fight
+// resolves. It must be a PURE, IDEMPOTENT function of run state, because the
+// turn engine rebuilds the enemy from the bestiary every single round
+// (partyCombatantsForSession) — the boss's numbers are re-derived on every
+// !attack. That is fine as long as the inputs are frozen for the duration of
+// the fight, which they are: the boss room is terminal, so no more nodes are
+// walked once the fight begins.
+//
+// In-combat Layer-2 hooks (Inversion Stitch, Amendment, Two Hearts) are a
+// separate seam — a new MonsterAbility.Effect case in applyAbility, shared by
+// both engines — and are not in this file.
+
+// applyBossRunModifiers folds any pre-combat Layer-2 mechanic for bossID into
+// the freshly-built enemy Combatant, reading the run the player took to get
+// here. It is dispatched by bestiary ID and is a no-op for every enemy that
+// isn't a hooked T6 boss, so the two build seams can call it unconditionally on
+// whatever they just built. A nil enemy or nil run is a no-op.
+func applyBossRunModifiers(bossID string, enemy *Combatant, run *DungeonRun) {
+	if enemy == nil || run == nil {
+		return
+	}
+	switch bossID {
+	case "boss_aurvandryx":
+		applyGreedTax(enemy, run)
+	}
+}
+
+// ── Greed Tax — Aurvandryx, the Ember Before Fire (First Hoard) ─────────────
+//
+// The First Hoard carries the game's richest LootBias nodes on purpose (the
+// gilded veins at 2.5–3.0; nowhere else in the game exceeds 1.0). Aurvandryx —
+// the hoard's true owner, not its guard dog — takes the interest out of your
+// hide: her Attack rises with how much of the gilded route you walked to reach
+// her cradle. Strip the veins and meet a furious wyrm; take the vow-of-poverty
+// line through the zone and fight her lean.
+//
+// Signal: the summed excess LootBias (above the 1.0 baseline) of every node the
+// run walked — NOT run.LootCollected, which only tracks the boss-only signature
+// manifest and is empty at the boss. A full-explore route through the First
+// Hoard accrues ~3.5 richness (+7 Attack); the cap covers a maximal line.
+//
+// Calibration (P8 sim sweep, L20 party+Pete+pets): the full-explore route lands
+// first_hoard at ~47% clear (mid-band 40–55), down from the taxless 56%. A lean
+// prod route pays no tax and fights her at the taxless rate; a maximal route
+// hits the cap. No base-stat retune was needed — the tax corrects the prior
+// slight overshoot into the band. NOTE: the sim's autopilot explores the whole
+// graph, so the sweep only exercises the full-route point; the lean/greedy
+// spread is a prod-only player-agency lever the headless sim cannot walk.
+const (
+	// greedTaxRichnessMult converts summed route richness into Attack points.
+	greedTaxRichnessMult = 2.0
+
+	// greedTaxMaxAttack caps the tax so a maximal hoard-run meets a very hard
+	// wyrm, never a literally-unbeatable one.
+	greedTaxMaxAttack = 12
+)
+
+// greedRouteRichness sums the excess LootBias (above the 1.0 baseline) of every
+// node the run has walked. Extracted for a deterministic unit test.
+func greedRouteRichness(run *DungeonRun) float64 {
+	g, ok := loadZoneGraph(run.ZoneID)
+	if !ok {
+		return 0
+	}
+	var rich float64
+	for _, id := range run.VisitedNodes {
+		if n, exists := g.Nodes[id]; exists && n.Content.LootBias > 1.0 {
+			rich += n.Content.LootBias - 1.0
+		}
+	}
+	return rich
+}
+
+// greedTaxAttack is the Attack surcharge for a route of the given richness.
+func greedTaxAttack(richness float64) int {
+	tax := int(richness * greedTaxRichnessMult)
+	if tax > greedTaxMaxAttack {
+		tax = greedTaxMaxAttack
+	}
+	if tax < 0 {
+		tax = 0
+	}
+	return tax
+}
+
+func applyGreedTax(enemy *Combatant, run *DungeonRun) {
+	enemy.Stats.Attack += greedTaxAttack(greedRouteRichness(run))
+}