Adds zone_underdark, zone_feywild_crossing, zone_dragons_lair, and
zone_abyss_portal flavor files with elite-room intros, boss-entry
pools (where canonical twinbee_gm_flavor.go didn't already ship one),
boss ability callouts, and zone-specific lore. Wires the four pools
into zoneRoomEntryPool / bossEntryPool / bossSignaturePool /
eliteRoomEntryPool / zoneLorePool, and replaces the now-obsolete
"unflavored zone" assertions with Tier 4-5 positive routing tests.
RoomEntryUnderdark, RoomEntryDragonsLair, BossEntryInfernax, and
BossEntryBelaxath are reused verbatim from twinbee_gm_flavor.go per
the canonical-flavor-first rule.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
