N7/E4: Seasonal events — 1-week holiday skins

Four anchor holidays (Hallowtide, Midwinter Feast, Sweethearts' Revel,
First Bloom) each get a 7-day window (anchor ±3 days) that layers three
things on the world, all reusing existing machinery:

  1. A themed Omen that overrides the weekly rotation. Reuses the B3 omen
     effect fields, so the non-combat rule holds; kept behind activeOmen's
     simOmenDisabled guard (and activeSeason honours it too) so no season
     path can reach the balance sim or move the golden.
  2. A curated curio shelf at Luigi's — existing registry items rotated to
     the front of dailyCuriosStock, so no net-new power enters the economy.
     Off-season output is byte-identical to before.
  3. A themed road visitor via the ambient seam — a season_visitor event
     that leaves a sellable keepsake + coin gift. No combat: the ambient
     seam still never opens a fight.

Pure function of the UTC date + anchor calendar — no schema, ticker, or
persistence, same discipline as the Omen and holiday calendar. Season
banner rides the existing morning DM (no net-new scheduled message).

go test ./internal/plugin ./internal/db green; combat golden byte-identical.

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

internal/plugin/adventure_render.go

@@ -318,6 +318,14 @@ func renderAdvMorningDM(userID id.UserID, equip map[EquipmentSlot]*AdvEquipment,
 	sb.WriteString(omenMorningLine(activeOmen()))
 	sb.WriteString("\n\n")
 
+	// N7/E4 — a live season adds a "what's live this week" banner under the Omen
+	// line (its themed omen already rode the line above). Same morning DM, no
+	// net-new scheduled message.
+	if s, ok := activeSeason(); ok {
+		sb.WriteString(seasonBannerLine(s))
+		sb.WriteString("\n\n")
+	}
+
 	// Pick a morning greeting
 	greeting, _ := advPickFlavor(MorningDM, userID, "morning_dm")
 	displayName, _ := loadDisplayName(userID)