docs: the 2026-09-28 round's implementation record, readings and self-review - #733

The record of the 2026-09-28 ecosystem round (mcpp 2026.9.28.2, #730 and #731; xlings 2026.9.28.2, openxlings/xlings#628), in .agents/docs/2026-09-28-ecosystem-design-and-optimisation-plan.md, now revision 4 and landed:

• §8.2 adds findings F7 (§7.4 named the wrong sibling of CI: the LLVM toolchain job reports success while mcpp's self-build with [email protected] fails #729; the lint's reading is seven W1 problems), F8 (the release gate's first reading was of its own harness, fixed in release canaries: run each command under the step's bash, by path #731) and F9 (a packaging revision reaches a consumer when its index does: the canaries, which restore CI's cached mcpp home, still built against qt-base revision 0).
• §8.3 records the before and after reading of every task; the three sandbox runs (15/12, 20/7 and 27/0 ok/failed for the previous pair, the new xlings alone, and the new pair); the per-row docs check; GalTranslPP on Windows three ways, including its CI without the [Feature] 统一 Windows MSVC ABI 的 CRT 选择，支持普通 clang++ 并同步标准库 BMI #718 workaround; and the index's full sweep with 2026.9.28.2 (22 shards green).
• §8.4 is the round's self-review.
• The CHANGELOG entry of 2026.9.28.2 names release canaries: run each command under the step's bash, by path #731.

Documentation only.