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/adventure_character.go

@@ -118,6 +118,18 @@ type AdventureCharacter struct {
 	// N5/D1c the finale reward-once flag. True after the first finale clear;
 	// overlay-read, written by the atomic markEpilogueClearedDB.
 	EpilogueCleared bool
+	// N7/B2 Renown — cumulative overflow XP earned past the L20 cap. Overlay-read
+	// from player_meta.renown_xp, written by the atomic addRenownXP, never the
+	// bulk character save. RenownLevel() derives the prestige level from it.
+	RenownXP int
+}
+
+// RenownLevel is the derived prestige level (renown_xp / renownXPPerLevel).
+func (c *AdventureCharacter) RenownLevel() int {
+	if c == nil {
+		return 0
+	}
+	return renownLevelFor(c.RenownXP)
 }
 
 type AdvEquipment struct {