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/combat_session_build.go

@@ -211,6 +211,13 @@ func (p *AdventurePlugin) partyCombatantsForSession(sess *CombatSession) ([]*Com
 	// until every seat is built.
 	applySeatWeights(players, levels, companions)
 
+	// Layer-2 pre-combat boss mechanics: fold in any run-state-derived
+	// adjustment (e.g. Aurvandryx's Greed Tax) before the party HP scaling. This
+	// is re-derived every round like everything else here; its inputs are frozen
+	// for a terminal boss fight, so the result is stable. No-op for every
+	// non-hooked enemy.
+	applyBossRunModifiers(monster.ID, &enemy, run)
+
 	// Party-only enemy HP bump, re-derived each turn from the template so it never
 	// compounds. Matches the scalar startPartyCombatSession used for the initial
 	// persist; solo (one seat, weight 1) scales by 1.0.