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_cmd.go

@@ -97,7 +97,7 @@ func (p *AdventurePlugin) handleFightCmd(ctx MessageContext) error {
 
 	// Seat the whole party, leader first. A solo player is a one-seat roster and
 	// takes the path they always took: one build, one INSERT, no participant rows.
-	seats, enemy, senderSkip, refusal := p.buildFightSeats(ctx.Sender, roster, monster, int(zone.Tier), run.DMMood)
+	seats, enemy, senderSkip, refusal := p.buildFightSeats(ctx.Sender, roster, monster, int(zone.Tier), run.DMMood, run)
 	if refusal != "" {
 		return p.replyDM(ctx, refusal)
 	}