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/dnd_expedition_cycle.go

@@ -400,6 +400,9 @@ func (p *AdventurePlugin) deliverBriefing(e *Expedition, now time.Time) error {
 
 	line := pickMorningBriefing(e.CurrentDay)
 	body := renderMorningBriefing(e, line, burn)
+	if sl := p.shadowBriefingLine(e); sl != "" {
+		body += "\n" + sl + "\n"
+	}
 	if restSummary != "" {
 		body += "\n💤 _" + restSummary + "_\n"
 	}
@@ -513,6 +516,9 @@ func (p *AdventurePlugin) deliverBriefingEventAnchored(e *Expedition, priorBrief
 
 	line := pickMorningBriefing(e.CurrentDay)
 	body := renderMorningBriefing(e, line, burn)
+	if sl := p.shadowBriefingLine(e); sl != "" {
+		body += "\n" + sl + "\n"
+	}
 	if forced {
 		body += "\n_The autopilot stalled overnight; the day rolled over without rest._\n"
 	}