test: add Simple8 scheduler tests

The Simple8 algorithm at org-drill-determine-next-interval-simple8 had no direct test coverage, completing the trio after SM2 (37 tests) and SM5 (32 tests, just landed). Adds a per-function test file with 34 tests across Normal, Boundary, Error, algorithm-verification, and helper-specific categories.

Simple8-specific surface gets dedicated coverage. The function returns a 6-element list (not 7 like SM2/SM5) and recomputes ease from meanq each call rather than carrying an EF parameter through. Failure does not increment totaln (different from SM2/SM5, which always increment). The four delta-days configurations (nil, positive × flag, negative × flag) each take a distinct code path, including a late-review use-n adjustment that SM5 doesn't have.

The three pure-math helpers (simple8-first-interval, simple8-interval-factor, simple8-quality->ease) get five direct tests so polynomial-coefficient typos can't drift silently.

All 34 pass on first run as characterization. Full suite at 214 of 214 (was 180, +34).

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