[git-cgallagher]

What

Adds Option A + C from the coin-visual discussion: a sidebar checkbox — Answer with coin pictures — visible only when the US currency worksheet type is selected. When on, subtraction ($a − $b =) and make-change problems replace the blank answer line with a write-in row of coin pictures: one of each denomination followed by a × ___ box, so the child expresses the answer as a coin combination.

• Bills at Grade 3+: the $1/$5 bill art joins the row where subtraction amounts reach $9.99; Grade 2 stays coins-only.
• Answer key: boxes are filled with the fewest-coins breakdown (greedy — provably optimal for US denominations), plus a key-only note that any combination with the correct total is correct, so valid alternate answers don't get marked wrong. The worksheet itself gets a one-line instruction above the grid.
• Total line kept (Total: $______) so the child computes the difference first, then represents it in coins.
• Hidden-state guard: genMixed strips coinAnswers — the checkbox is invisible on mixed review, so a stale checked state can't change mixed output (WYSIWYG).
• coinSVG/billSVG gain an optional scale param (row renders at 0.55×, viewBox untouched) to keep the default 12-problem sheet compact.
• Count-the-money and addition problems are unchanged; easy difficulty is unaffected (count-only, matching existing gating).

Docs (same-PR rule)

• CHANGELOG.md — Unreleased → Added entry. No TYPE_META change, so the wizard needs nothing (it reads TYPE_META live).

Testing

• node --check generator.js app.js (the CI gate) passes.
• Generator smoke test, grades 2–5 × easy/medium/hard × 40 problems: every fewest-coins breakdown sums exactly to its answer; bills excluded at G2 / included at G3+; zero coin-row problems on easy, with the flag off, and across 30 seeded mixed-review sheets (10 of which drew currency sections).

Notes (non-blocking)

• At the default 12 problems the coin rows make blocks taller; small icons (0.55×) keep it close to one page, but very high counts may spill to a second printed page — same as other tall layouts.
• On easy difficulty the checkbox has no visible effect (easy is count-only by design).

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