Document Mac memory tiers through 64 GB

[skyzh]

Why

Learners choose a course checkpoint before they run the model. The existing table skipped several unified-memory configurations sold across recent Apple-silicon Mac mini and MacBook models, including 24 GB and 48 GB.

What changed

• list every distinct recent Mac mini/MacBook unified-memory tier through 64 GB: 8, 16, 18, 24, 32, 36, 48, and 64 GB
• add conservative Week 1–4 model recommendations for each tier without expanding the course-supported model set
• clarify the dense Week 2 boundary and cite official Apple specifications for the hardware tiers

Verification

• git diff --check
• mdbook build book
• book/sitemap.sh --check
• rendered preface table and footnotes inspected
• all four official Apple specification links resolve

mdbook test book remains broadly red on the repository's existing unlabelled non-Rust code fences across unchanged chapters; the changed preface produces no test failure.

Authored-by: GPT-5.6 Sol + Cindy