Week 2 Day 2: packed W4 decode matvec

[skyzh]

Scope

Extract Week 2 Day 2 from the reviewed five-day Draft #316 onto the merged Day 1 course. The active lesson teaches packed W4 weights and the quantized-matvec checkpoint after the Day 1 capacity cache. Starter, reference code, supplied tests, CLI, benchmarks, profiler, and shipped-day CI expose the Day 2 contract while reserving later kernels and checkpoint names for later PRs.

The lesson retains the signed-affine W4 derivation, eight-nibble packing, BF16 metadata, Apple bandwidth and roofline example, matched benchmark method, and historical Week 2 URLs. Readable text accompanies the displayed equations. The preface identifies Day 1 as the dense control and Day 2 as the packed path; the 4B comparison is optional.

Local evidence

• Both learner and reference native extensions build. The Day 2 shipped-day gate collected 351 tests: 350 passed, 1 skipped. Day 1 and Day 2 reference files passed 6/6 and 13/13. Copied learner tests collect and stop at named TODO seams.
• Installation, documented model-free command parsing, mdBook, sitemap, rendered local links, lint, diff and Git integrity checks passed on the integrated local head. The two-page documentation correction was checked again on its direct successor.
• Independent local reviews on the corrected head fabba526 returned factual/preservation PASS, math/command correctness GO, learner WALKABLE, and static accessibility PASS. These verdicts are local evidence; this public PR still requires fresh exact-head reviews.

Open gates

Hosted macOS run 35950765030 succeeded on exact head fabba526: both native extensions built and the shipped-day manifest collected 351 tests (349 passed, 2 skipped). The runner's skip count differs from the local author run's one skip.

This PR is Draft. Fresh public-head factual, correctness, learner, and accessibility reviews and all current repository gates remain required before Ready or merge. No Qwen model inference or performance benchmark was run for this Day 2 slice, and this PR makes no speed claim. The separate full-course Draft #316 performance screen does not validate Day 2 performance.

The current main-branch workflow deploys GitHub Pages on push. Merging this PR would trigger publication, so the Day 2 publication decision remains separate from the approval used for Day 1. No release, tag, settings, visibility, or later-day work is part of this PR.