refactor: collapse secureOverwriteBuffer to pattern-fill only

[jacaudi]

Summary

• Delete the dead useRandom=true async random-fill branch from secureOverwriteBuffer in crypto-storage.js — the sole caller already passed false.
• Collapse the function from 27 lines (3-arg with async cooperative yield) to 7 lines (2-arg synchronous pattern fill).
• Add 3 unit tests (16-byte aligned, zero-length, non-divisible-by-4) verifying the FF 00 FF 00 pattern output.

Test plan

• docker run --rm -v $(pwd):/work -w /work node:22-alpine sh -c "node scripts/build.js && node --test app/tests/test-*.js" — 169/169 passing
• No useRandom references remain in src/
• Both secureOverwriteBuffer references (definition + caller) use 2-arg form
• Security-hardening tests still pass

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