Skip prefetching anchors with the download attribute

[sankalpsthakur]

Summary

• Do not observe / prefetch <a download> links by default (boolean or named download attribute)
• Applies in both listen (src/index.mjs) and the chunks path (src/chunks.mjs)
• Adds a fixture + puppeteer test and a short README note under Custom Ignore Patterns

Fixes #181

Why this matters

Download links often target large binaries (PDFs, archives, exports). Prefetching them wastes bandwidth and cache space on paths the user may never follow — especially on metered or Save-Data connections where quicklink already throttles prefetch. Treating download like other built-in ignore patterns (external origins, rel=nofollow, etc.) keeps prefetch focused on navigational routes.

Test plan

• npm test (19/19 passing)
• npm run size (within limits)
• npm run lint
• Confirm CLA check passes for @sankalpsthakur

[google-cla]

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[sankalpsthakur]

CLA signed for @sankalpsthakur / sankalphimself@gmail.com. cla/google is green.