📝 T022: Note mpay PlaocService IPC envelope

Author: Gaubee

openspec/changes/add-dweb-authorize/proposal.md

@@ -106,10 +106,19 @@ The concrete runtime entrypoint is still unconfirmed. We must lock it down with
 - Candidate A: mpay-style `plaoc.initAppPlaocEventListener((eventId, url, event) => ...)`
 - Candidate B: `@plaoc/plugins` `dwebServiceWorker.addEventListener('fetch', (event) => ...)` (request/response IPC)
 
+#### Observed mpay PlaocService (evidence only)
+
+The legacy mpay implementation strongly suggests Candidate B is real in practice:
+
+- Wallet listens via `dwebServiceWorker.addEventListener('fetch', ...)` and generates an internal `eventId` like `eventId:${Date.now()}` to correlate UI with the captured fetch event.
+- `getCallerAppInfo(eventId)` reads `event.getRemoteManifest()`.
+- `respondWith(eventId, pathname, payload)` serializes JSON `{ "data": payload }` with `application/json`.
+- `removeEventId(eventId)` responds with JSON `{ "data": null }`.
+
 ### Event ID Format
 
-- **Source**: Generated by DWEB runtime when triggering authorization
-- **Format**: TBD (treat as opaque identifier). Note: if Candidate B is used, the underlying request correlation id is a numeric `reqId`.
+- **Source**: Wallet-side listener generates an internal eventId (used to correlate UI flow with the underlying IPC event)
+- **Format**: Opaque string (mpay uses `eventId:${Date.now()}`); if Candidate B is used, an underlying numeric `reqId` may exist but does not need to be exposed
 - **Lifetime**: Valid for 5 minutes from creation
 - **Single-use**: Each eventId can only be responded to once
 
@@ -139,6 +148,7 @@ interface CallerAppInfo {
 | `/wallet/authorize/signature` | `{ signature: string, txHash?: string }` | `{ error: 'rejected' \| 'timeout' \| 'insufficient_balance' }` |
 
 **Compatibility note (mpay)**:
+- mpay wire response uses JSON `{ "data": <payload> }` with `application/json` (and `removeEventId` responds `{ "data": null }`).
 - mpay may send/expect raw array payloads (e.g. address returns `addressList` array; signature returns a result array aligned by request index).
 - Phase B MUST confirm with runtime samples whether KeyApp should normalize these shapes at the adapter boundary (recommended for smooth mpay migration).