feat(books): add ABCs paperback link

[konjoinfinity]

Summary

The Learning ABCs with Squish paperback is now published on Amazon (https://www.amazon.com/dp/B0HLVYYDW8).

• Adds a Paperback chip to the ABCs row in the formats list. It links to B0HLVYYDW8, next to the existing Ebook chip (B0HLTVNZBP).
• Removes "Learning ABCs with Squish is Kindle only for now." from the formats intro, since it's no longer true.
• The row price stays at $3.99. Like the other rows, it shows the ebook price.

I checked locally at 390px and 1280px: every book row now has Ebook and Paperback chips, and the page doesn't scroll sideways. Amazon blocked automated fetching, so the ASIN comes from the author's link and hasn't been checked against the live page.

Type of change

• Bug fix
• New feature
• Performance improvement
• Documentation update
• Refactor / cleanup

Checklist

• pytest tests/ passes locally: not run, static HTML only
• ruff check squish/ tests/ reports no errors: not run, no Python touched
• No hardcoded absolute paths
• No model weights, eval output files, or log files staged for commit
• Changes are scoped to one logical concern
• Performance-sensitive changes include a before/after squish bench run: N/A

Related issues

Follow-up to #243 and #244.

🤖 Generated with Claude Code

https://claude.ai/code/session_01R3pogbkBhhhr9h7qHxBWXf

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[konjoinfinity]

G4 · Complexity + Size + DRY is red, but this PR didn't cause it. The failing step is the 500-line file gate on squish/backend.py (511 lines, not in .konjo/oversized-allowlist.txt). This PR only changes docs/books/index.html. main fails the same way, as did #243 and #244.

main also fails two other checks this PR's test jobs will hit. One is tests/test_wave120_dead_global_purge.py::test_server_line_count_under_threshold, because server.py is exactly 6400 lines against a < 6400 ceiling. The other is three macOS-only bfloat16 KV-cache tests that raise 'bfloat16' is not a valid PEP 3118 buffer format string. None of these has a fix yet, and all are outside this PR's scope. I didn't re-run anything, because these checks fail deterministically.

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