0.2.6

fix(bootstrap): resumable, self-verifying browser download with retries

The 560 MB CloakBrowser archive reliably truncates on flaky international
links to GitHub's CDN, producing a corrupt zip that fails extraction with
"ZIP bad CRC" on every entry. The previous guard just deleted the file and
asked the user to retry, which loops forever on such links.

Rework download() into a real downloader:
- Resumable via HTTP Range (CDN advertises Accept-Ranges: bytes) — a
  transfer that dies at 95% resumes the last 5%, not the whole 560 MB.
- SHA-256 verified from disk against CloakBrowser's published SHA256SUMS;
  a mismatch (proxy/AV rewriting the stream) forces a fresh full re-pull.
- Bounded auto-retry (5 attempts, linear backoff) before surfacing an
  actionable error suggesting a network change or the CFT engine.
- Attempt 1 always starts clean, scrubbing any stale .partial on disk.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>