feat: Phase 2D — orchestrator, arguments/verdict endpoints, fabric announce

State machine driver + camp allocator + judge-submitted verdicts +
broadcast hook to Fabric announce channel.

internal/orchestrator/
- allocator.go: pure function implementing the 3-camp rule from the
  2026-05-23 design session — for each camp (pro/con/judge), random
  pick from volunteers; backfill unfilled camps from remaining
  unallocated signups if pool is large enough; <3 final → cancel
  with diagnostic reason. rng injected for test determinism.
- allocator_test.go: 7 tests covering empty/insufficient/single-volunteer
  /multi-volunteer-no-dup/backfill/insufficient-backfill/large-pool
  distinctness invariants. All pass.
- ticker.go: scans every 15s (configurable via ORCHESTRATOR_TICK_INTERVAL),
  drives 3 state transitions atomically:
    created → signup_open (post fabric announcement async)
    signup_open → signup_closed | cancelled (run allocator, write camps)
    signup_closed → debating (open round 0)
  debating → completed is driven by the verdict POST handler (the
  implicit "judging" sub-state is captured by the gate
  status==debating AND now>=debate_end_at). Per-topic transitions
  use SELECT FOR UPDATE so concurrent ticker instances are safe.

internal/fabric/announce.go: HTTP client posting to a Guild announce
channel using x-fabric-system-key header (the Phase 1 gate). Wraps
the formatted topic announcement (title/summary/timing/schema). All
4 config fields required to enable; any missing → no-op with log
(orchestrator runs fine without Fabric coupling for dev).

internal/store/{round,camp,argument,verdict}_store.go: CRUD layer
for the remaining v2 entities. CampStore.WriteAllocation accepts a
tx so the orchestrator can wrap allocator+camps+status into one
atomic transition.

internal/httpapi/handlers/arguments.go:
- POST /api/topics/{id}/arguments — agent posts during debate. Gates:
  agent must be in a camp on this topic; status==debating; content
  nonempty and <=32KB; attached to latest open round.
- GET /api/topics/{id}/arguments — full transcript, visibility-gated.

internal/httpapi/handlers/verdict.go:
- POST /api/topics/{id}/verdict — judge submits. Gates: caller==judge
  camp; status==debating AND now>=debate_end_at; verdict valid JSON;
  rationale required. On success: writes verdicts row (unique on
  topic_id → 409 on dup) and flips topic.status to completed.
- GET /api/topics/{id}/verdict — visibility-gated.

config: 5 new env vars — FABRIC_GUILD_BASE_URL,
FABRIC_ANNOUNCE_CHANNEL_ID, FABRIC_SYSTEM_API_KEY,
FABRIC_BOT_BEARER_TOKEN, ORCHESTRATOR_TICK_INTERVAL.

routes.go: wired new handlers — POST signups/arguments/verdict gated
on agent bearer; GET arguments/verdict on optional-auth chain
(public topics readable anonymously).

main.go: instantiates announcer + ticker; ticker.Run in a goroutine
sharing the lifetime ctx.

go vet + gofmt clean; 7/7 allocator tests pass; 12M static binary.

Next: Phase 2E (deploy to t3 with nginx + CF origin cert) or
Phase 2D.5 (SSE stream for live transcript subscribers).

internal/store/verdict_store.go

@@ -0,0 +1,63 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+)
+
+type Verdict struct {
+	ID           string    `db:"id" json:"id"`
+	TopicID      string    `db:"topic_id" json:"topic_id"`
+	JudgeAgentID string    `db:"judge_agent_id" json:"judge_agent_id"`
+	VerdictJSON  []byte    `db:"verdict_json" json:"-"` // surface raw via Render
+	Rationale    string    `db:"rationale" json:"rationale"`
+	TokensInput  int       `db:"tokens_input" json:"tokens_input"`
+	TokensOutput int       `db:"tokens_output" json:"tokens_output"`
+	ProducedAt   time.Time `db:"produced_at" json:"produced_at"`
+}
+
+type VerdictStore struct {
+	db *sqlx.DB
+}
+
+func NewVerdictStore(db *sqlx.DB) *VerdictStore { return &VerdictStore{db: db} }
+
+type SubmitVerdictInput struct {
+	TopicID      string
+	JudgeAgentID string
+	VerdictJSON  []byte
+	Rationale    string
+	TokensInput  int
+	TokensOutput int
+}
+
+// Submit writes the (one-and-only) verdict for a topic. Unique constraint
+// on topic_id means a second submission returns a duplicate-key error;
+// caller surfaces that as 409.
+func (s *VerdictStore) Submit(ctx context.Context, in SubmitVerdictInput) (*Verdict, error) {
+	id := uuid.NewString()
+	if _, err := s.db.ExecContext(ctx,
+		`INSERT INTO verdicts (id, topic_id, judge_agent_id, verdict_json, rationale, tokens_input, tokens_output)
+		 VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		id, in.TopicID, in.JudgeAgentID, in.VerdictJSON, in.Rationale, in.TokensInput, in.TokensOutput); err != nil {
+		return nil, err
+	}
+	return s.GetByTopic(ctx, in.TopicID)
+}
+
+func (s *VerdictStore) GetByTopic(ctx context.Context, topicID string) (*Verdict, error) {
+	var v Verdict
+	err := s.db.GetContext(ctx, &v, `SELECT * FROM verdicts WHERE topic_id = ?`, topicID)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &v, nil
+}