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/db/db.go

@@ -393,6 +393,14 @@ func runMigrations(d *sql.DB) error {
 		// character save, so a monotonic false→true flag can't be clobbered.
 		// DEFAULT 0 correct for every pre-existing row, so no bootstrap.
 		`ALTER TABLE player_meta ADD COLUMN epilogue_cleared INTEGER NOT NULL DEFAULT 0`,
+		// N7/B2 Renown (gogobee_engagement_plan.md §B2). At the L20 cap, overflow
+		// XP that grantDnDXP used to drop instead accumulates here as cumulative
+		// prestige XP. renown_xp is monotonic and written by an atomic += (the
+		// journal_pages pattern), never the bulk character save; renown_level is
+		// DERIVED from it (renown_xp / renownXPPerLevel) rather than stored, so
+		// there is no read-modify-write race and no second field to disagree.
+		// DEFAULT 0 == "no renown", correct for every pre-existing row, no bootstrap.
+		`ALTER TABLE player_meta ADD COLUMN renown_xp INTEGER NOT NULL DEFAULT 0`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {