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/argument_store.go

@@ -0,0 +1,62 @@
+package store
+
+import (
+	"context"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+
+	"git.hangman-lab.top/hzhang/Dialectic.Backend/internal/models"
+)
+
+type Argument struct {
+	ID       string      `db:"id" json:"id"`
+	TopicID  string      `db:"topic_id" json:"topic_id"`
+	RoundID  string      `db:"round_id" json:"round_id"`
+	Camp     models.Camp `db:"camp" json:"camp"`
+	AgentID  string      `db:"agent_id" json:"agent_id"`
+	Content  string      `db:"content" json:"content"`
+	PostedAt time.Time   `db:"posted_at" json:"posted_at"`
+}
+
+type ArgumentStore struct {
+	db *sqlx.DB
+}
+
+func NewArgumentStore(db *sqlx.DB) *ArgumentStore { return &ArgumentStore{db: db} }
+
+type PostArgumentInput struct {
+	TopicID string
+	RoundID string
+	Camp    models.Camp
+	AgentID string
+	Content string
+}
+
+func (s *ArgumentStore) Post(ctx context.Context, in PostArgumentInput) (*Argument, error) {
+	id := uuid.NewString()
+	if _, err := s.db.ExecContext(ctx,
+		`INSERT INTO arguments (id, topic_id, round_id, camp, agent_id, content)
+		 VALUES (?, ?, ?, ?, ?, ?)`,
+		id, in.TopicID, in.RoundID, in.Camp, in.AgentID, in.Content); err != nil {
+		return nil, err
+	}
+	var a Argument
+	if err := s.db.GetContext(ctx, &a, `SELECT * FROM arguments WHERE id = ?`, id); err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+// ListByTopic returns the full transcript in posted order. Used by the
+// judge agent at end-of-debate to write the verdict, by the frontend
+// to render, and by observer agents querying via plugin.
+func (s *ArgumentStore) ListByTopic(ctx context.Context, topicID string) ([]Argument, error) {
+	var rows []Argument
+	if err := s.db.SelectContext(ctx, &rows,
+		`SELECT * FROM arguments WHERE topic_id = ? ORDER BY posted_at ASC, id ASC`, topicID); err != nil {
+		return nil, err
+	}
+	return rows, nil
+}