Core runtime depends on Node Buffer despite documented edge portability

Summary

The README states that CallScript supports serverless and edge environments, but the core package currently relies on the Node-specific global Buffer.

The affected locations are:

• packages/callscript/src/expr/eval.ts — Base64 and Base64URL helpers
• packages/callscript/src/execute.ts — tool-result byte measurement
• packages/callscript/src/runner.ts — digest-output byte measurement

In a web-standard runtime where globalThis.Buffer is unavailable, Base64 expressions fail, and ordinary tool results can turn an otherwise successful run into an error.

Reproduction

Using the built package:

import { callscript, evalExpr, tool } from "callscript";

globalThis.Buffer = undefined;

evalExpr('Base64.encode("hi")', {});
// TypeError: Cannot read properties of undefined (reading 'from')

const engine = callscript({
  tools: [
    tool({
      name: "demo.echo",
      execute: async () => ({ ok: true }),
    }),
  ],
});

const result = await engine.run({
  script: {
    steps: [
      {
        id: "result",
        call: "demo.echo",
        args: {},
      },
    ],
  },
});

console.log(result.status);
// "error"

The execution error is caused by attempting to call Buffer.byteLength.

Expected behavior

Core functionality should work in edge and browser-like runtimes without requiring a Buffer polyfill.

Base64 helpers should also preserve their current behavior, including Unicode and Base64URL support.

Proposed fix

Replace the affected Buffer usages with dependency-free, web-standard UTF-8 and Base64 utilities, and add regression tests that run with Buffer unavailable.