feat(agentad): AgentAd Marketplace PRD + reference exchange (M5)

Add the AgentBBS M5 "AgentAd marketplace" spec and a working reference
implementation built on the existing @logicsrc/schemas AgentAd contracts.

- docs/agentad-marketplace.md: two-sided exchange PRD (buy/sell sides,
  match -> auction -> pace -> serve -> meter -> settle, CoinPay settlement,
  AgentBBS as reference publisher, milestones M5.0-M5.5).
- packages/agentad (@logicsrc/agentad): reference exchange
  - builders that emit schema-valid, always-disclosed ad/campaign/placement docs
  - HMAC-signed, single-use impression/click tracking tokens
  - AgentAdExchange: targeting/format/category matching, second-price auction,
    budget pacing + daily caps, frequency capping, token-driven metering
  - pluggable settlement (InMemorySettlement) that can't overspend escrow
  - runtime validation against the canonical agentad-*.schema.json
- 22 vitest cases (builders, tokens, full serve/meter/settle lifecycle).
- Wire package into root build; link the PRD from README + docs/agentad.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/agentad/src/index.ts

@@ -0,0 +1,41 @@
+// @logicsrc/agentad — AgentAd Marketplace reference exchange.
+// See docs/agentad-marketplace.md (AgentBBS milestone M5) and docs/agentad.md.
+
+export * from "./types.js";
+export {
+  AGENTAD_VERSION,
+  createAd,
+  createCampaign,
+  createPlacement,
+  createAdRequest,
+  type NewAd,
+  type NewCampaign,
+  type NewPlacement,
+  type NewAdRequest
+} from "./builders.js";
+export {
+  validate,
+  assertValid,
+  agentAdSchemas,
+  type AgentAdSchemaKind,
+  type ValidationResult
+} from "./validate.js";
+export {
+  mintToken,
+  verifyToken,
+  type TokenKind,
+  type TokenPayload,
+  type VerifyResult
+} from "./tokens.js";
+export {
+  InMemorySettlement,
+  type SettlementProvider,
+  type InMemorySettlementOptions
+} from "./settlement.js";
+export {
+  AgentAdExchange,
+  type AgentAdExchangeOptions,
+  type RequestOptions,
+  type ConfirmImpressionResult,
+  type LedgerEntry
+} from "./exchange.js";