docs(libgbm): mark section 8.1 superseded at its own heading

Section 2's C2 already carried a supersession banner; section 8 did not, so a
reader landing there directly would act on the retired B3 conclusion. The
reading guide said so, but a guide only helps someone who started at the top.

Scoped to 8.1 rather than the whole section: 8.2-8.6 (the untested-alternative
finding, the V-matrix correction, the R2a/R2b/R2c split) all still stand, and
8.1's own measurement is still fact -- it just proves "do not install into the
project scope" rather than "layer over it".

Author: Sunrisepeak

.agents/docs/2026-08-30-gbm-cross-repo-closed-loop-plan.md

@@ -355,6 +355,12 @@ version `GLIBC_2.43' not found (required by …/libgallium-25.0.7.so)
 
 ## 8. 深度自我 review(2026-08-30 第二轮)
 
+> **⚠ 本节的 8.1(结论:mcpp 侧应采用「分层继承 B3」)已被 §12.1 推翻。** 实现时测出:
+> 装到**全局 scope** 之后 sysroot 天然看得见,B3 不需要。8.1 那张「项目 subos 缺
+> libgcc_s/libstdc++」的表仍是事实,但它证明的是「别往项目 scope 装」,不是「需要叠加」。
+> 8.2–8.6 的其余内容仍然成立。**mcpp 侧怎么修,以 §12.1 为准。**
+
+
 第一轮方案有四处实质错误。逐条记录,因为其中三条是「没做实验就下结论」。
 
 ### 8.1 B1/B2 是假二分,正确形态是**分层继承**(B3)