Exercise 1: Three Siblings, Running-Sum Stacking — Possible Solution
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THE TEST
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    x1 = block({'height': '15px'})
    x2 = block({'height': '25px'})
    x3 = block({'height': '5px'})
    container = block({}, children=[x1, x2, x3])

    layout_block(container, root_containing_block)   # root width 300px
    x1.dimensions.content.y
    x2.dimensions.content.y
    x3.dimensions.content.y

RESULT
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    x1.content.y -> 0
    x2.content.y -> 15
    x3.content.y -> 40

WHY x3's OWN POSITION REFLECTS BOTH EARLIER HEIGHTS, NOT JUST x2's
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layout_block_children walks the children in order, and after EACH one
finishes, grows the container's own accumulated content.height:

    for child in layout_box.children:
        layout_block(child, d)
        d.content.height += child.dimensions.margin_box().height

Before x1 is laid out, d.content.height is 0 -- calculate_block_position
computes x1's own y as cb.y + cb.height = 0 + 0 = 0. After x1 finishes,
d.content.height becomes 0 + 15 = 15 (x1's own margin-box height, with
no margin/border/padding of its own, equals its 15px explicit height
exactly).

Before x2 is laid out, d.content.height is now 15 -- x2's own y is
computed as cb.y + cb.height = 0 + 15 = 15. After x2 finishes,
d.content.height becomes 15 + 25 = 40.

Before x3 is laid out, d.content.height is now 40 -- x3's own y is
cb.y + cb.height = 0 + 40 = 40. This is exactly x1's 15px PLUS x2's
25px, not just x2's own 25px alone -- because d.content.height is a
single running total that every prior child has already contributed
to, not a value that gets reset or that only remembers the most recent
child.

WHY THIS WORKS AS AN ANSWER
------------------------------
The container's own `d.content.height` field is doing double duty: by
the time all children are laid out, it's also the container's OWN
final content height (used later by calculate_block_height if height
is 'auto') -- but DURING the loop, at any given moment, it's simply
"how much vertical space has been consumed by children processed SO
FAR." Reading it at the START of each child's own position calculation
is what makes stacking cumulative rather than pairwise -- each child
sees the true running total, not just its immediate predecessor's own
height.