Week 2 Day 3: SIMD matrix prefill

[skyzh]

Scope

Add Week 2 Day 3 SIMD matrix prefill on top of the merged Day 2 W4 course. The active lesson introduces the simd-matmul checkpoint, a 10×97 partial-tile operator exercise, and a public model-output comparison after the completed Day 1–2 checkpoints. The learner-owned native SIMD seam remains unfinished in the starter; reference code and supplied tests provide the runnable control. CLI, benchmark selectors, and shipped-day CI expose Day 3 while later-day routes remain deferred.

The chapter explains the 32×32 cooperative tile, four 16×16 quadrants, guarded edge stores, readable equation alternative, and matched measurement method. Former Week 2 URLs remain historical. Required measurement uses the setup-cached 0.6B model; 4B is optional. This PR makes no speed claim.

Local evidence

• Both learner and reference extensions build; installation checks pass. The Day 3 shipped-day gate collected 369 tests: 368 passed, 1 optional-model skip. Direct Day 1–3 reference files passed 25 tests.
• Supplied Task 1 compares public 1×3 outputs on the fallback path. Task 2 checks the 10×97 SIMD operator against the readable packed control. Task 3 uses fixed tiny fixtures with seeds 0 and 4, compares public 1×10 checkpoint BF16 outputs and full shapes with separate caches at atol=0.75, rtol=0.05, and restores MLX RNG state. The unfinished learner starter reaches its named SIMD TODO in Task 2; cumulative model tasks still require Days 1–2 completion.
• mdBook, sitemap, rendered links, chapter command parsing, lint, diff hygiene, and Git integrity checks passed on local head 49bda24c. Fresh independent local reviews returned factual/preservation PASS, correctness/public-contract GO, learner WALKABLE, and static accessibility PASS. These are local verdicts only.

Open gates

Hosted macOS run 35969966892 succeeded on exact head 49bda24c: both native extensions built and the shipped-day manifest collected 369 tests (367 passed, 2 skipped). The runner has one more skip than the local author run; independent CI review will interpret the difference.

This PR is Draft. Fresh independent reviews of this exact public head, base, and body and all current repository gates remain required before Ready or merge. No cached Qwen benchmark was run for this Day 3 slice; the separate full-course Draft #316 performance screen does not establish Day 3 speed.

Chi approved normal website updates for this five-day Week 2 batch. The ordinary main-branch Pages workflow may run after a separately gated merge. This PR does not authorize a release, tag, settings or visibility change, manual deployment, or later-day work.