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/validate.ts

@@ -0,0 +1,72 @@
+// Runtime validation against the canonical AgentAd JSON Schemas. Mirrors the
+// approach in @logicsrc/validators: import the schema documents directly from
+// @logicsrc/schemas and compile them with Ajv 2020. Keeping this local (rather
+// than importing the built @logicsrc/validators dist) lets the exchange and its
+// tests run straight from source with no cross-package build step.
+
+import * as Ajv2020Module from "ajv/dist/2020.js";
+import * as addFormatsModule from "ajv-formats";
+import type { ErrorObject } from "ajv";
+
+import adSchema from "../../schemas/schemas/agentad-ad.schema.json" with { type: "json" };
+import placementSchema from "../../schemas/schemas/agentad-placement.schema.json" with { type: "json" };
+import adRequestSchema from "../../schemas/schemas/agentad-ad-request.schema.json" with { type: "json" };
+import adResponseSchema from "../../schemas/schemas/agentad-ad-response.schema.json" with { type: "json" };
+import impressionSchema from "../../schemas/schemas/agentad-impression.schema.json" with { type: "json" };
+import clickSchema from "../../schemas/schemas/agentad-click.schema.json" with { type: "json" };
+import campaignSchema from "../../schemas/schemas/agentad-campaign.schema.json" with { type: "json" };
+
+type CompiledValidator = { (data: unknown): boolean; errors?: ErrorObject[] | null };
+const Ajv2020 = (Ajv2020Module as unknown as {
+  default: new (options: Record<string, unknown>) => { compile: (schema: unknown) => CompiledValidator };
+}).default;
+const addFormats = (addFormatsModule as unknown as { default: (ajv: unknown) => void }).default;
+
+export const agentAdSchemas = {
+  "agentad-ad": adSchema,
+  "agentad-placement": placementSchema,
+  "agentad-ad-request": adRequestSchema,
+  "agentad-ad-response": adResponseSchema,
+  "agentad-impression": impressionSchema,
+  "agentad-click": clickSchema,
+  "agentad-campaign": campaignSchema
+} as const;
+
+export type AgentAdSchemaKind = keyof typeof agentAdSchemas;
+
+export type ValidationResult =
+  | { ok: true; kind: AgentAdSchemaKind; data: unknown }
+  | { ok: false; kind: AgentAdSchemaKind; errors: ErrorObject[] };
+
+const cache = new Map<AgentAdSchemaKind, CompiledValidator>();
+
+function compiled(kind: AgentAdSchemaKind): CompiledValidator {
+  const existing = cache.get(kind);
+  if (existing) return existing;
+
+  const ajv = new Ajv2020({ allErrors: true, strict: true });
+  addFormats(ajv);
+  const fn = ajv.compile(agentAdSchemas[kind]);
+  cache.set(kind, fn);
+  return fn;
+}
+
+export function validate(kind: AgentAdSchemaKind, data: unknown): ValidationResult {
+  const fn = compiled(kind);
+  const ok = fn(data);
+  return ok
+    ? { ok: true, kind, data }
+    : { ok: false, kind, errors: fn.errors ?? [] };
+}
+
+/** Validate or throw with a readable message. Returns the value narrowed to T. */
+export function assertValid<T>(kind: AgentAdSchemaKind, data: T): T {
+  const result = validate(kind, data);
+  if (!result.ok) {
+    const detail = result.errors
+      .map((e) => `${e.instancePath || "/"} ${e.message ?? "is invalid"}`)
+      .join("; ");
+    throw new Error(`Invalid ${kind} document: ${detail}`);
+  }
+  return data;
+}