[joshpitzalis]

Summary

Check a complete letter attempt against its reference trace when the learner presses Check, regardless of stroke order, direction, pen lifts, or retracing.

draw → Check → reference coverage + stray-ink check
                 ├─ pass: retain the drawing and show success
                 └─ retry: retain editable ink and give simple reasons
new drawing → clear the previous result
Clear / Retry → reset the attempt

Stacked on implement/strokes; preserves the existing writing-feature layout.

Evidence

• Before: drawing only recorded gestures. New tests failed for the absent Check control and exposed a dot-only false pass.
• After: pnpm check, pnpm test (2 fence, 21 content/session/checker, 15 browser), and pnpm build passed after merging the base branch. Commit and merge hooks remained enabled.
• Tests cover missing parts, stray ink, correction, retained learner ink, resize, resets, stale results, and direction/order/retracing invariance. Generated near-threshold cases cover sampling and segmentation; an intentional missing-coverage defect was detected and removed.
• Independent standards review passed. Spec review found one pad-edge tolerance issue; the test-first fix and follow-up spec review passed. Regression tests cover small off-pad marks, accumulated exterior/interior ink, and huge finite excursions.

Merge Danger

Door: two-way

Blast Radius: practice

Margins are provisional and uncalibrated; the original reference remains marked review pending. No persistence, infrastructure, or deployment changes. Physical devices have not been checked.

Closes #7