docs: add v0 design spec for FSRS scheduler

I drafted a design spec for adding FSRS (Free Spaced Repetition Scheduler) as a fourth choice in org-drill-spaced-repetition-algorithm. It pins FSRS-4.5 as the version target, documents the DSR state model, the rating mapping from org-drill's 0-5 quality to FSRS's 1-4 scale, the update equations, the new DRILL_FSRS_* properties, and the integration shape into the existing scheduler dispatch.

The spec is v0 with six explicit DECIDE: markers carrying recommended defaults: version pin, parameter tuple, quality mapping, return shape, cold-start UI, and equation cross-check. The companion todo entry stays open at todo.org:125 until those land and implementation begins. The file moves to fsrs-spec.org at the project root once the DECIDEs resolve.

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