FSRS-7 ADR Support

[JSchoreels]

Hello, this is based on #395 and #404 .

Ofc #395 is a dependency but at least if some people want to try it out, they can also here.

CleanShot.2026-06-02.at.14.30.58.mp4

[gemini-code-assist]

Code Review

This pull request introduces FSRS-7 model support alongside FSRS-6, splitting model-specific math, inference, parameter clipping, and training logic into version-specific modules. It changes the representation of elapsed time (delta_t) from u32 to f32 to support fractional days and same-day reviews. Additionally, it introduces the Cost ADR (Active Desired Retention) policy framework for optimizing desired retention based on study time and memorization targets, complete with simulation paths, training configs, and examples. The review feedback correctly points out a portability issue in a new example file where a specific user's absolute path is hardcoded as the default collection path.

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[gemini-code-assist]

Hardcoding an absolute path to a specific user's home directory (/Users/jschoreels/...) breaks portability and will cause the example to fail on other machines unless --collection is explicitly provided. Consider using a relative path to a standard test fixture (like tests/data/collection.anki21) as the default.

const DEFAULT_COLLECTION: &str = "tests/data/collection.anki21";