Make benchmark --standalone reports portable offline

[corner123]

benchmark --standalone currently produces a page that still loads Alpine.js from a CDN and refers to screenshots by filesystem path. Opening the report without network access leaves the task list empty; moving it away from the recording breaks its images.

This change embeds a pinned Alpine.js build and local screenshots when --standalone is set. It retains the existing CDN and path behavior for regular reports, raises a clear error if a screenshot cannot be included, and documents the portable output and its size tradeoff. The vendored Alpine.js license is included in both the package and generated report.

Validation:

• Added a browser regression that blocks HTTP requests, moves the HTML, removes its source screenshot, then checks the task list, step view, and image. Both failure modes were reproduced before the fix; the test passes afterward.
• Added coverage for missing screenshots, relative paths, and the CLI path through benchmark data.
• uv run pytest tests/ -q: 163 passed, 79 skipped. The browser test ran locally with Chromium; it skips on hosts where the Playwright browser is not installed.
• uv run ruff check . and uv build passed; the built wheel contains the pinned script and its license.