Exercise 3: Not Even a Cross-Ecosystem Question Here — Possible Solution
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CONFIRMING bcryptjs VERIFIES THE $2y$ HASH
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Given the legacy admin's real stored hash (a $2y$-tagged bcrypt hash
produced by PHP's password_hash()), calling
bcrypt.compare(plainPassword, legacyHash) with the correct plaintext
password returns true, with no configuration change of any kind
needed on the bcryptjs side.

WHY THIS ISN'T A CROSS-ECOSYSTEM QUESTION HERE
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Per this chapter, the Rails rebuild's own Chapter 9 had to verify
this carefully because it was a genuine cross-ecosystem case - a hash
generated by PHP's password_hash() being checked by Ruby's own
bcrypt gem, two entirely different languages and libraries. Here,
bcryptjs is the exact same npm package the Next.js rebuild's own
Chapter 9 already used to verify the identical legacy hash - there is
no second language or ecosystem involved at all, just the same
JavaScript library doing the same job a second time within this
series.

WHY THIS WORKS AS AN ANSWER
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It correctly confirms bcryptjs verifies the $2y$-tagged hash
correctly, and correctly explains why this chapter treats that as an
unremarkable repeat of an already-established fact rather than a
new cross-ecosystem finding the way Rails' own Chapter 9 had to.