fix(build): copy schema.json to dist and externalize zod (#534)

Two packaging bugs:

1. The './schema.json' export pointed to dist/src/generated/schema.json
   but nothing copied it there (tsc is emitDeclarationOnly, Bun.build
   doesn't emit JSON). Consumers got ERR_MODULE_NOT_FOUND.
   Fix: cpSync after tsc runs.

2. zod is a peerDependency but was bundled into every entry point
   (~270KB each). This causes dual-instance bugs where consumer-side
   'instanceof ZodError' and schema.extend() break across boundaries.
   Fix: add zod to external list for all non-with-deps entries.

Bundle size impact:
  app.js          293952 -> 24561  (-91.6%)
  app-bridge.js   299517 -> 30112  (-89.9%)
  react/index.js  295558 -> 26153  (-91.2%)
  server/index.js 291500 -> 22135  (-92.4%)
  app-with-deps   321814 -> 321814 (unchanged, intentional)

build.bun.ts

@@ -1,9 +1,15 @@
 #!/usr/bin/env bun
 import { $ } from "bun";
+import { cpSync, mkdirSync } from "node:fs";
 
 // Run TypeScript compiler for type declarations
 await $`tsc`;
 
+// Copy schema.json (tsc is emitDeclarationOnly, Bun.build doesn't emit JSON assets).
+// Needed for the "./schema.json" package export.
+mkdirSync("dist/src/generated", { recursive: true });
+cpSync("src/generated/schema.json", "dist/src/generated/schema.json");
+
 const isDevelopment = Bun.env.NODE_ENV === "development";
 
 // Build all JavaScript/TypeScript files
@@ -25,10 +31,14 @@ function buildJs(
   });
 }
 
+// zod is a peerDependency — keep it external so consumers share a single
+// zod instance (instanceof ZodError / schema.extend() break with duplicate copies).
+const PEER_EXTERNALS = ["@modelcontextprotocol/sdk", "zod"];
+
 await Promise.all([
   buildJs("src/app.ts", {
     outdir: "dist/src",
-    external: ["@modelcontextprotocol/sdk"],
+    external: PEER_EXTERNALS,
   }),
   buildJs("src/app.ts", {
     outdir: "dist/src",
@@ -36,19 +46,19 @@ await Promise.all([
   }),
   buildJs("src/app-bridge.ts", {
     outdir: "dist/src",
-    external: ["@modelcontextprotocol/sdk"],
+    external: PEER_EXTERNALS,
   }),
   buildJs("src/react/index.tsx", {
     outdir: "dist/src/react",
-    external: ["react", "react-dom", "@modelcontextprotocol/sdk"],
+    external: ["react", "react-dom", ...PEER_EXTERNALS],
   }),
   buildJs("src/react/index.tsx", {
     outdir: "dist/src/react",
-    external: ["react", "react-dom", "@modelcontextprotocol/sdk"],
+    external: ["react", "react-dom", ...PEER_EXTERNALS],
     naming: { entry: "react-with-deps.js" },
   }),
   buildJs("src/server/index.ts", {
     outdir: "dist/src/server",
-    external: ["@modelcontextprotocol/sdk"],
+    external: PEER_EXTERNALS,
   }),
 ]);