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

Exact identity and scope

• base: ed91403011258bfbe0f8b4dfadc9d310d9b7ef2d
• head: a05e9dfc246c4f92079483aa8fd67de1c146552d
• tree: 5f8bdd484810335c21e8dcdd108fef4a9cc9b708
• changed path: only book/src/preface.md, 19 additions / 8 removals

Verification and review

• git diff --check
• mdbook build book
• book/sitemap.sh --check
• rendered preface table and footnotes inspected
• all four official Apple specification links resolve
• hosted macOS run 33237131454 completed successfully on the exact head
• fresh factual/preservation review: GO
• fresh learner review: GO
• Chi accepted the table on the exact frozen head

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.

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