Address review: MCPB build, guard the SDK's own handlers, doc fixes

- pdf-server build-mcpb.mjs packs the ext-apps build from this checkout
  into the staging dir instead of asking the registry for ^2.0.0, which
  is not published yet; CI's "Build MCPB bundle" step failed on ETARGET.
- Claim notifications/cancelled and notifications/progress in the
  MethodRegistry right after super() so a direct setNotificationHandler
  cannot silently replace the base Protocol's own handlers.
- Migration guide and add-app-to-server skill: custom methods use the
  three-argument setRequestHandler("m", { params }, handler) form (the
  two-argument form is only for spec methods); zod schemas live in
  @modelcontextprotocol/core, not client/server; it is
  toolInfo.tool.outputSchema that loosened in schema.json, and the
  recursive JSON-value def is nested under three definitions; core is a
  required peer that client already depends on.
- app-bridge.ts: fix "forthe"/"forresult" typos in @see links, update
  three JSDoc examples to the method-string setNotificationHandler form,
  move the UntypedHandlerSetter type below the import block (same in
  app.ts).
- wire-compat.test.ts: say the fixtures are 1.x-shaped, not captures.

docs/migrate-to-2.md

@@ -27,7 +27,7 @@ host-side deltas are in error responses (see below).
 | CDN / `*-with-deps` | nothing extra: `./app-with-deps` and `./react-with-deps` bundle client, core and zod (about 25% larger than the 1.x bundles) |
 
 `@modelcontextprotocol/client` is a required peer (`App` and `AppBridge` extend
-its `Protocol` class); `@modelcontextprotocol/core` is installed transitively;
+its `Protocol` class); `@modelcontextprotocol/core` is a required peer that `client` already depends on, so npm installs it without you listing it;
 `@modelcontextprotocol/server` stays optional and is only needed for the
 `./server` helpers. Node.js 20+ is required.
 
@@ -48,8 +48,10 @@ its `Protocol` class); `@modelcontextprotocol/core` is installed transitively;
   `extra.mcpReq.id`.
 - **`setRequestHandler` / `setNotificationHandler` take method names.**
   `app.setRequestHandler(SomeRequestSchema, handler)` becomes
-  `app.setRequestHandler("some/method", handler)` (pass the params schema as the
-  optional extra argument for validation).
+  `app.setRequestHandler("some/method", { params: SomeParamsSchema }, (params, ctx) => …)`
+  for custom methods (the handler receives the parsed params); the two-argument
+  `setRequestHandler("tools/call", handler)` form exists only for spec-defined
+  method names.
 - **Errors.** Remote JSON-RPC errors are `ProtocolError` (numeric `code`);
   local conditions are `SdkError` with a string `code`: request timeout →
   `"REQUEST_TIMEOUT"`, connection closed → `"CONNECTION_CLOSED"`. Cancelling a
@@ -74,10 +76,15 @@ The published `./schema.json` export is regenerated from the 2.x core schemas:
 
 - `McpUiToolResultNotification.params.structuredContent` is any JSON value (was
   `type: "object"`).
-- Result `_meta` documents `io.modelcontextprotocol/serverInfo` and no longer
-  lists `progressToken` / `related-task` (both still pass through).
-- `McpUiHostContext.toolInfo.tool.inputSchema` is a loose object.
-- A recursive `$defs.__schema0` JSON-value definition is added.
+- `McpUiToolResultNotification.params._meta` documents
+  `io.modelcontextprotocol/serverInfo` and no longer lists `progressToken` /
+  `related-task` (both still pass through).
+- `McpUiHostContext.toolInfo.tool.outputSchema` is a loose object (only
+  `$schema` is documented); `inputSchema.properties` values are now typed as
+  JSON values.
+- A recursive JSON-value definition (`__schema0`) is added under the `$defs` of
+  `McpUiHostContext`, `McpUiHostContextChangedNotification` and
+  `McpUiInitializeResult`.
 
 ## Checklist
 
@@ -87,7 +94,8 @@ The published `./schema.json` export is regenerated from the 2.x core schemas:
    `@modelcontextprotocol/server`, `sdk/server/streamableHttp.js` →
    `NodeStreamableHTTPServerTransport` from `@modelcontextprotocol/node`,
    `sdk/server/stdio.js` → `@modelcontextprotocol/server/stdio`, `sdk/types.js`
-   → `@modelcontextprotocol/client` or `server`).
+   → `@modelcontextprotocol/client` or `server` for the types,
+   `@modelcontextprotocol/core` for the zod schemas).
 3. Wrap raw zod shapes with `z.object({...})`.
 4. Update custom handlers to the `extra.mcpReq.*` context and method-keyed
    `setRequestHandler` calls.