feat(server): createWebhookEmitter + createAdcpServer.webhooks integration

[bokelley]

Publisher-side webhook emission — the symmetric counterpart to PR #629's receiver dedup. Closes the "we haven't spun up an actual server and watched the full stack verify" gap flagged during #631 review, and the `createWebhookEmitter` follow-up listed in that PR's body.

What

createWebhookEmitter (`src/lib/server/webhook-emitter.ts`)

One call — `emit({ url, payload, operation_id })` — and the emitter handles:

• RFC 9421 signing with a fresh nonce per attempt (adcp#2423).
• Stable `idempotency_key` per `operation_id` reused across retries (adcp#2417) — regenerating on retry is the AdCP Testing Framework with Authentication, Hierarchical Logging & Security Features #1 at-least-once-delivery bug the runner-side conformance suite catches.
• Serialize once, sign once, post once. JSON with compact separators (`,` / `:`, no spaces), byte-identical bytes feed both the `content-digest` input and the HTTP body. Closes the Python `json.dumps` default-spacing trap pinned by adcp#2478.
• Retry with exponential backoff + jitter on 5xx / 429. Terminal on 4xx and on 401 responses carrying `WWW-Authenticate: Signature error="webhook_signature_*"` — retrying a signature failure produces identical bytes and identical rejection.
• Pluggable `WebhookIdempotencyKeyStore` (default in-memory). Production publishers with multi-replica emitters swap in a durable backend the same way `AsyncHandlerConfig.webhookDedup` accepts one on the receiver side.
• HMAC-SHA256 / Bearer fallback for legacy buyers that registered `push_notification_config.authentication.credentials`. HMAC path uses the same compact-separators pinning.

`createAdcpServer` integration

New `webhooks?: { signerKey, retries?, idempotencyKeyStore?, … }` config option. When set, `ctx.emitWebhook` is populated on every handler's context:

```ts
createAdcpServer({
name, version,
webhooks: { signerKey: { keyid, alg: 'ed25519', privateKey: jwk } },
mediaBuy: {
createMediaBuy: async (params, ctx) => {
const media_buy_id = await persist(params);
await ctx.emitWebhook({
url: params.push_notification_config.url,
payload: { task: { task_id, status: 'completed', result: { media_buy_id } } },
operation_id: `create_media_buy.${media_buy_id}`,
});
return { media_buy_id, packages: [] };
},
},
});
```

When `webhooks` is omitted `ctx.emitWebhook` is `undefined`, so handlers that don't emit don't need the option.

Tests

• 14 unit tests (`test/lib/webhook-emitter.test.js`): happy path + 9421 round-trip through our own verifier; retry on 503/429 with stable key + fresh nonce; terminal 4xx; terminal 401 with `WWW-Authenticate: Signature`; max-attempts cap; cross-call stability (same operation_id → same key); malformed-generator rejection; HMAC + Bearer fallback headers; observability hooks fire with correct attempt numbers.
• 3 full-stack E2E tests (`test/lib/webhook-emitter-server-e2e.test.js`): `createAdcpServer` with a real handler → `ctx.emitWebhook` → real HTTP POST → receiver captures → `verifyWebhookSignature` accepts. No mocks on the signer or verifier path. Also covers the `ctx.emitWebhook = undefined` omission case and cross-emit idempotency-key stability within a single handler invocation.

All 4194 tests pass / 0 fail / 7 annotated skips.

Exports

From `@adcp/client/server`:

• `createWebhookEmitter`, `memoryWebhookKeyStore`
• Types: `WebhookEmitter`, `WebhookEmitterOptions`, `WebhookEmitParams`, `WebhookEmitResult`, `WebhookEmitAttempt`, `WebhookEmitAttemptResult`, `WebhookIdempotencyKeyStore`, `WebhookRetryOptions`, `WebhookAuthentication`
• `HandlerContext.emitWebhook` — new optional field, populated when `webhooks` config is set.

Follow-ups

• Brand.json → JWKS auto-resolver. Receiver-side helper so `webhook_signing: { brand: 'acme.example' }` works without callers hand-building a `JwksResolver`. Separate PR; the current `StaticJwksResolver` / `HttpsJwksResolver` already cover the direct-URL case.
• Observability helpers — today `onAttempt` / `onAttemptResult` give raw data; a thin metric emitter (retry-count, terminal-errors, latency histograms) could be a small follow-up.

Changeset

`.changeset/webhook-emitter.md` — `minor`. Pure additive: new export surface, new optional `HandlerContext` field, new optional config option. Nothing breaks.