Exercise 3: Why ForEach Requires Identifiable or an Explicit id — Possible Solution
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ForEach's own real job is more than just looping over an array once to
draw some initial views - it has to keep tracking each individual
element across real, later data updates too, so that when the
underlying array changes (an item added, removed, or reordered),
SwiftUI can correctly figure out which on-screen row corresponds to
which specific real data element, animate insertions/removals
correctly, and avoid confusing one row's own identity with another's.

Doing that correctly requires a real, STABLE way to identify each
element - some value that reliably refers to "this particular real
item," independent of its current position in the array (since
position alone isn't reliable once items get inserted, removed, or
reordered). The Identifiable protocol supplies exactly that: a real id
property the type itself guarantees is present and stable. An explicit
id: \.someProperty argument tells ForEach to use a specific existing
property as that same kind of stable identifier instead, when the
type doesn't conform to Identifiable directly.

A plain struct with neither Identifiable conformance nor an explicit
id: argument gives ForEach no real way to satisfy this requirement at
all - there's no property SwiftUI could safely use as a stable
identity, and rather than silently falling back to something
unreliable (like array position, which would break the moment the
data changes), Swift's own compiler catches this as a real error at
compile time, since ForEach's own initializer's generic constraints
simply aren't satisfied by a type offering no identifiable property
whatsoever.

ANSWER: ForEach needs a stable way to identify each element across data
updates - not just for an initial render, but to correctly track
insertions, removals, and reordering afterward. Identifiable supplies
that via a guaranteed id property; an explicit id: argument supplies it
via an existing property instead. A plain struct with neither satisfies
neither of ForEach's own required initializer signatures, so the
compiler rejects it at compile time rather than falling back to an
unreliable identity source like array position.

WHY THIS WORKS AS AN ANSWER
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This explains the real underlying reason ForEach needs stable identity
(tracking changes over time, not just an initial draw) rather than
treating the Identifiable requirement as an arbitrary rule to memorize.