feedback1 integrate: DESIGN §21.21-§21.25 and CHANGELOG for batch 6 and the PR-credited fixes

DESIGN maps F1-F5 in the §21 intro, adds the hammer-per-deployment row (user 2026-10-03) to
§21.20 with the batch's official rules and [ASSUMED] items, and rewrites the normative lines:
§3 fenced tiles, §5.3 liftoff, §5.4 fatal / ignoreSelect / redeploy, §5.5 bodies, blocking and
enemy selection, §6.4 onBuy and the item snapshot, §8.2 down row/col, §18.3, §19.7. CHANGELOG
0.1.1, SIM, DATA and PLAYING follow; docs-consistency checks them against the code.

docs/DATA.md

@@ -3,6 +3,8 @@
 All files in `data/` except `data/assets.json` are produced by **`node tools/build-data.mjs`** (task F1) from the
 official zh_CN client data ([Kengxxiao/ArknightsGameData](https://github.com/Kengxxiao/ArknightsGameData)) joined
 with `docs/research/*.json`. Do not edit them by hand — change the build script and rebuild.
+`data/assets.json` is written by `tools/fetch-assets.mjs`, which keeps the current file rather than drop entries whose
+downloads failed on this machine unless `--allow-shrink` (or `--prune`) is passed (docs/ASSETS.md, DESIGN §21.25).
 
 ```
 node tools/build-data.mjs              # build (downloads missing official files into .cache/gamedata/)