fix(drill-deck): cut leakage false positives and codify source/date conventions

Health ran the new leakage check on a 43-card deck and hit two false-positive classes. The check read the whole card body, so a =Source: <label> — <url>= citation line inflated the front/back overlap whenever the URL slug repeated the question's words. Range/category cards ("What are the HbA1c ranges across normal, prediabetes, and diabetes?") tripped it too, because the question's categories echo in the answer even though the recalled content is the numbers.

drill-deck-stats.py now routes leakage through an is_leaky helper. It strips =Source:= and created-date lines before computing overlap, and exempts a card when the answer carries a numeric range or threshold the question lacks. leakage_ratio itself is unchanged, so the genuine-restatement case still flags.

Two body conventions now hold: a =Source:= citation goes at the end of a card after two blank lines, and no created/added date goes on a card. drill-to-anki.py now strips =Created:= / =:CREATED:= lines from the back as a backstop, and the workflow's Phase C removes them from the source during the rewrite. I added tests for the source-strip, the numeric carve-out, and the created-line strip, and documented all of it in drill-deck-review.org.

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