N6/D3: the Shadow — a simulated rival adventurer

A per-player NPC rival who "runs" the same zone progression on a midnight
ticker at ~1.3x the player's own clear pace, staying just ahead so it's a
race you can always see and nearly catch. Pure theatre: no combat, no
punishment, only race pressure and two payoffs at each zone clear.

- New adventure_shadow table, deliberately OUTSIDE the player_meta save
  fan-out so a character save can never clobber the ticker's advance (the
  isolation journal_pages earns by being grant-only, made structural). No
  bootstrap: absent row == no Shadow, minted lazily on first advance.
- midnightReset advances every player's Shadow once per UTC day (own
  idempotency guard); lead-capped so it never runs >2.5 zones ahead. When it
  clears a zone the player hasn't, it leaves a journal page waiting (D1 tie-in).
- Morning-briefing race-pressure one-liners (TwinBee voice, deterministic).
- Zone-clear payoff in finalizeExpeditionOnZoneClear: a bonus-XP crow when the
  player got there first, or the Shadow's waiting page when it did.
- !adventure shadow status view.

Review fixes (3 finders + verify) folded in before commit:
- Crow XP is now set-once per zone (crowed_mask), so re-running a zone the
  Shadow hasn't reached can't farm it.
- The waiting page is granted BEFORE the pending bit is retired, so a transient
  grant failure leaves the debt for the next clear instead of swallowing a page.
- The crow line no longer claims "+XP" when the grant errored.

Combat golden byte-identical (Shadow never touches SimulateCombat); go
build/vet/test green repo-wide.

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

internal/plugin/adventure.go

@@ -535,6 +535,8 @@ func (p *AdventurePlugin) dispatchCommand(ctx MessageContext) error {
 		return p.handleVaultCmd(ctx, strings.TrimSpace(args[len("vault"):]))
 	case lower == "journal":
 		return p.handleJournalCmd(ctx)
+	case lower == "shadow":
+		return p.handleShadowCmd(ctx)
 	}
 
 	return p.SendDM(ctx.Sender, "Unknown command. Type `!adventure help` to see available commands.")
@@ -567,6 +569,7 @@ const advHelpText = `**Adventure Commands**
 ` + "`!adventure mastery`" + ` — Per-slot equipment mastery progress and active bonus
 ` + "`!adventure treasures`" + ` — List your treasures · ` + "`treasures lock`" + ` to refuse swaps
 ` + "`!adventure journal`" + ` — Read the campaign pages you've recovered
+` + "`!adventure shadow`" + ` — See how your rival's run compares to yours
 ` + "`!hospital`" + ` — Visit St. Guildmore's Memorial Hospital (same-day revival when dead)
 ` + "`!thom`" + ` — Visit Thom Krooke (housing and loans)
 ` + "`!adventure help`" + ` — This message