Challenge 1: Where Three Specific Capstone Pieces Came From — Possible Solution
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(a) THE Repository<T> TEMPLATE comes from cpp2-1 (Templates). That
    chapter introduced generic class templates and compile-time
    monomorphization; the capstone's Repository<T> is a direct
    application of that exact material, written once and usable for
    any element type, not specific to Shape at all.

(b) THE EXCEPTION THROWN FROM Circle's CONSTRUCTOR comes from cpp2-6
    (Exception Handling). That chapter covered throw/try/catch and
    RAII's role in exception safety; Circle's constructor validating
    its radius argument and throwing std::invalid_argument on invalid
    input is a direct, real application of that material -- exactly
    the kind of "fail loudly rather than silently accept bad state"
    pattern that chapter established.

(c) SORTING SHAPES BY AREA combines TWO chapters together: cpp2-3 (The
    STL — Iterators & Algorithms), which introduced std::sort as a
    generic algorithm working through iterators regardless of
    container type, and cpp2-7 (Lambda Expressions), which introduced
    the [](const auto &a, const auto &b) { ... } comparator syntax
    passed directly as std::sort's third argument -- the capstone's
    sorting step is a direct, combined application of both chapters at
    once.

WHY THIS WORKS AS AN ANSWER
------------------------------
This correctly attributes each piece to its actual origin chapter
(including correctly identifying (c) as combining two chapters rather
than one), and briefly restates what each chapter specifically taught
that the capstone is now applying, confirming genuine understanding of
how the pieces connect rather than just naming chapter numbers.