N7/B2: Renown — prestige past the L20 cap

Overflow XP that grantDnDXP used to drop at L20 now accumulates as Renown
on player_meta.renown_xp (cumulative, atomic +=; renown_level derived as
renown_xp/25000). The reward is prestige-only: a derived rank ladder
(Renowned→…→Eternal), a cosmetic ✦N marker on the sheet and leaderboard,
a games-room shout on rank promotion, and !level progress once capped.

Renown perks are the two combat-neutral economy levers only — +loot / +XP,
capped at a streak-30 grant's economic half (+15% / +20%). combat_stats.go
reads DeathModifier/SuccessBonus/ExceptionalBonus (which map to Defense/
Attack/CritRate) but never LootQuality/XPMultiplier, so renown pays out even
through loadCombatBonuses without moving the golden or the balance corpus.
The plan's "-death penalty" perk is deliberately dropped (it would inflate
Defense).

The overflow→renown conversion and the character save commit in one
transaction (saveDnDCharacterExec now takes an executor), so a crash can
neither drop the overflow nor double-credit it on the next grant.

Schema: renown_xp column, DEFAULT 0 correct for every existing row, no
bootstrap (journal_pages/epilogue_cleared pattern).

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

internal/plugin/dnd_xp.go

@@ -127,15 +127,26 @@ func (p *AdventurePlugin) grantDnDXP(userID id.UserID, amount int) ([]LevelUpEve
 		events = append(events, LevelUpEvent{NewLevel: c.Level, HPGain: gain})
 	}
 
-	// Cap at L20 — overflow XP is silently dropped.
-	if c.Level >= dndMaxLevel {
+	// N7/B2 Renown — persist the character; at the cap, overflow XP that used to
+	// be dropped converts to Renown in the SAME transaction as the save, so a
+	// crash can neither lose the overflow nor double-credit it on the next grant.
+	// Renown is title + cosmetic + capped loot/XP perks, never combat stats, so
+	// the balance corpus stays valid.
+	var renownFrom, renownTo int
+	if c.Level >= dndMaxLevel && c.XP > 0 {
+		overflow := c.XP
 		c.XP = 0
-	}
-
-	if err := SaveDnDCharacter(c); err != nil {
+		from, to, err := saveDnDCharacterWithOverflow(c, overflow)
+		if err != nil {
+			return events, err
+		}
+		renownFrom, renownTo = renownLevelFor(from), renownLevelFor(to)
+	} else if err := SaveDnDCharacter(c); err != nil {
 		return events, err
 	}
 
+	p.announceRenown(userID, renownFrom, renownTo)
+
 	if len(events) > 0 {
 		// Phase 10 SUB3a-ii — Battle Master Relentless (L15) raises the
 		// superiority cap from 4 → 5; reconcile any level-gated subclass pool