feat: persist a session log for the stats dashboard

I added the persist + recording layer that the future stats dashboard reads, plus the v0 design spec at working/stats-dashboard/stats-dashboard.org that scoped the work. Every completed (non-suspended) drill session now appends one org-drill-session-record to a persisted org-drill-session-log via persist-defvar, mirroring the SM5-matrix pattern that's already in place. A new sibling defgroup org-drill-statistics groups the dashboard's customs.

The record carries: start-time, end-time, scope, algorithm, qualities (as a vector), pass-percent, new-count, mature-count, failed-count, cram-mode. Scope and algorithm are captured at session-start time onto two new org-drill-session slots (scope-at-start, algorithm-at-start) so a mid-session defcustom flip doesn't misrepresent what was actually drilled. Both org-drill--prepare-fresh-session and the org-drill-again resume path populate the slots.

I extracted pass-percent into org-drill--compute-pass-percent and call it from both org-drill-final-report and org-drill-session-record-from-session, so the user-visible report and the persisted record can't drift on the rounding or the failure-quality threshold.

I wrapped recording in condition-case at the call site, not ignore-errors. Any recorder bug or persist-save IO failure gets messaged so silent data loss leaves a forensic trail.

Corrupt-load recovery follows the SM5 path's condition-case fallback and adds an org-drill--session-log-quarantine helper that renames the bad file to a .corrupt-YYYY-MM-DDTHHMMSS sibling so the next save doesn't overwrite it. The seconds-granularity suffix prevents a same-day double corruption from clobbering the earlier quarantine. The helper depends on persist--file-location, an internal symbol guarded behind fboundp and documented inline.

The spec at working/stats-dashboard/stats-dashboard.org is ratified, with all 10 originally-open decisions resolved in the Ratified Decisions table at the bottom: persist-defvar, warn-and-rename corrupt-load recovery, quadrant-block sparklines, a single buffer-wide filter, CSV with proper quoting, sync dashboard open, defer aborted-session recording, the q/g/e/s/r/a/RET keymap, leech-quality threshold default 2.5, and a sibling org-drill-statistics defcustom group. The remaining work (dashboard renderer, CSV export, docs) lands in follow-up commits.

I followed TDD throughout. 24 tests in tests/test-org-drill-session-record.el cover struct construction, the shared pass-percent helper, the builder including scope/algorithm capture and mature-count summing, log appending with newest-first ordering and persist-save call-through, the symbol-bound smoke check, the quarantine rename plus its seconds-granularity contract, and the end-message hook (records on normal completion, skips on suspend, logs to message on recorder error). Full make test-unit green. eask compile clean.

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