Challenge 2: Fix the Mutable Default — Possible Solution
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Given (buggy):
    has 'tags' => (is => 'rw', default => []);

Fixed:
    has 'tags' => (is => 'rw', default => sub { [] });

One-sentence explanation:
default => [] builds exactly ONE arrayref at attribute-declaration
time, which every instance that doesn't supply its own tags would
then share and mutate together, exactly like Course 1, Chapter 2's
mutable-default-argument gotcha resurfacing in Moo's own attribute
system.

WHY THIS WORKS AS AN ANSWER
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The fix reuses the chapter's own warn-box correction directly and
unchanged — wrapping the empty arrayref in sub { [] } instead of
writing default => [] on its own defers building the arrayref until
each individual object is actually constructed, per the chapter's own
explanation, giving every instance a genuinely independent, fresh
reference rather than one shared one.

The explanation reuses the chapter's own root-cause framing almost
word for word: "the exact same root cause as Course 1, Chapter 2's
mutable-default-argument gotcha, resurfacing in a new form" — a
single mutable object gets created once and shared where independent
copies were actually intended, the identical underlying mistake this
course has now shown surfacing in two genuinely different contexts
(a plain subroutine parameter back in Course 1, and now a Moo
attribute declaration here), reinforcing that it's one general
principle rather than two unrelated rules to memorize separately.