N5/D1c: the finale — !expedition start epilogue

Closes the Hollow King arc with a solo one-shot boss fight, reached via
`!expedition start epilogue` (intercepted before the zone/loadout
machinery). Unlock: all 24 journal pages found + both Tier-5 zones cleared.

- Encounter: runZoneCombat + renderBossOutcome, the arena's own pattern —
  no supplies, no walk, no party, no new mechanics. The stat block is
  Belaxath's (the abyss boss whose gate "lets one thing come home")
  re-dressed as the King returned in full, HP ×1.25 for a capstone; the
  ability/AC/CR carry over unchanged.
- Reward-once: first clear grants a unique title ("Kingsbane") + one
  Legendary + a games-room notice; later clears are a flavour-only
  rematch. The flag is a dedicated player_meta.epilogue_cleared column,
  latched by an atomic UPDATE (never the bulk save) so the monotonic
  false→true can't be clobbered — same discipline as D1a's journal
  bitmask. A loss costs a death, as any boss fight does.
- Title is written after a fresh character reload so runZoneCombat's
  post-fight HP persist isn't overwritten (the survivalist-milestone
  gotcha).

Golden byte-identical; go test ./... green.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_character.go

@@ -115,6 +115,9 @@ type AdventureCharacter struct {
 	// == page i+1). Read-only overlay from player_meta.journal_pages; writes go
 	// through the atomic grantJournalPageDB, never the bulk character save.
 	JournalPages int64
+	// N5/D1c the finale reward-once flag. True after the first finale clear;
+	// overlay-read, written by the atomic markEpilogueClearedDB.
+	EpilogueCleared bool
 }
 
 type AdvEquipment struct {