Add hybrid script cancellation for browserjs() with cooperative + V8 termination

[hjanuschka]

Summary

Implements reliable script cancellation for browserjs() using a hybrid dual-strategy approach:

Technical Details

Why Cooperative Cancellation Works

By wrapping the Promise constructor, we inject cancellation checks into every promise creation:

window.Promise = function (executor) {
  return new OriginalPromise((resolve, reject) => {
    if (window.__sitegeist_cancelled) {
      reject(new Error("Script execution was cancelled"));
      return;
    }
    
    const wrappedResolve = (value) => {
      if (window.__sitegeist_cancelled) {
        reject(new Error("Script execution was cancelled"));
      } else {
        resolve(value);
      }
    };
    
    executor(wrappedResolve, reject);
  });
};

This means any await becomes a cancellation checkpoint automatically!

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Related Work

• Chromium CL 7110745: "extensions: Add per-execution termination to userScripts.execute() API"
• Analysis: https://gist.github.com/hjanuschka/80d1e6a8b8e9fabfb702522f76857561
• Demo repo: https://github.com/hjanuschka/script_cancel_demo

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Migration Guide

No migration needed! Existing code automatically benefits from cooperative cancellation. For long synchronous loops, optionally add explicit yields:

// Before (still works, but better with yield):
await browserjs(async () => {
  for (let i = 0; i < 10000; i++) {
    // Synchronous work
    processItem(i);
  }
});

// After (recommended for loops > 100 iterations):
await browserjs(async () => {
  for (let i = 0; i < 10000; i++) {
    processItem(i);
    
    // Add yield every 50 iterations
    if (i % 50 === 0) await __sitegeist_yield();
  }
});