Week 2 Day 5: tiled attention and selected checkpoint

[skyzh]

Scope

Add Week 2 Day 5 tiled dense-attention prefill and the cumulative selected checkpoint on merged Day 4. Learners implement the BF16/D128 native tiled path for query length at least 9, with causal/additive masks and grouped-query attention. Short or unsupported cases use the readable fallback. The selected checkpoint exposes capacity, RMSNorm, and tiled-attention controls independently. Required matched measurement uses the setup-cached Qwen3-0.6B model; 4B is optional. This PR makes no speed claim.

The lesson makes Days 1–5 active and qualifies older Week 2 URLs that were reused by active lessons. The repaired Day 1–2 supplied tests check public cache, quantized-operator, and packed-model behavior instead of private layout, source text, or cache movement counters; the Day 2 lesson describes that observable scope.

Local evidence

• Frozen head 360e485860b387ba7ecdc32bf24113cb269a7798, tree 7b1159092980fa82ff8a453aea259ba829747c54, descended from merged Day 4 main 68642f05df1094ca20f92779504f737921a8dbd9. The cumulative diff is exactly 55 paths, each matched to an accepted source blob. The final integration commit changes only the two repaired supplied-test files on the corrected docs parent.
• Both native extensions and installation passed. The full unfiltered reference suite passed 679 tests with 2 optional skips; direct Day 1–5 selectors passed 6/14/6/7/10. Focused Day 5, tiled-boundary, interface, and repaired Day 1–2 tests passed in independent review. Copied learner tests reach named native and preceding-day TODOs.
• mdBook/sitemap rendered 49 pages; the final-tree local scan found 1,909 links and 993 fragments with no broken targets. The 39 lesson Bash blocks passed syntax checks, and nine focused selectors collected the intended tests. Changed-file lint/format, diff hygiene, and strict Git checks passed.
• Four fresh independent local reviews on this exact head returned factual/preservation PASS, correctness/test GO, learner WALKABLE (conditional on completing Days 1–4), and static-accessibility PASS. These are local verdicts only. No product benchmark or real-model speed result is claimed.

Open gates

This PR is Draft. Hosted macOS full unfiltered reference CI succeeded on this exact head (681 collected, 677 passed, 4 optional model skips in Week 1 Day 5 and Week 3 Day 1); the required 0.6B tests ran. Four fresh independent reviews of the exact public head, base, and current body remain required before Ready or ordinary merge. The full-course Draft #316 remains source material; its verdicts do not transfer.

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