fix(ci): pin the floor 2026.9.7.1, not the tip, because the tip has a scanner regression

The pin moved to 2026.9.10.2 on the reasoning that #361 took the current
release. CI disagreed: `mysql-connector-cpp` failed on linux default,
linux llvm AND macOS, deterministically, while every other member passed.

It is not that package's fault, and not a dependency problem. mcpp's
scanner errors on an ordinary block comment in libmysqlclient's
`sql-common/client_plugin.cc`. Reduced to four lines:

    /*
      module (exe)
    */
    int main() { return 0; }

    error: scanner errors:
      src/main.cpp:2: '(exe) ...

A line inside a block comment is read as a module declaration when the
comment's opener sits on its own line -- the same text on one line
(`/* module (exe) */`) scans fine, and `module x` without the parenthesis
scans fine either way. So a comment that merely uses the English word
"module" before a parenthesis breaks the scan.

Bisected across releases with that file:

    2026.8.27.2  OK          2026.9.7.1   OK
    2026.9.6.3   OK          2026.9.10.2  scanner errors

The regression therefore lands AFTER 2026.9.7.1 -- which is precisely the
release that first carries `mcpp::package_name()`, the API huxerui's build
program needs. The floor and the last good version are the same release,
so this takes the floor.

Verified at 2026.9.7.1, both members that mattered:

    mcpp test -p huxerui-module       test result ok. 1 passed
    mcpp test -p mysql-connector-cpp  test result ok. 1 passed

Reported upstream. Move to the tip once the scanner is fixed -- and per
the batching note above, that move should carry any other descriptor
waiting on a newer engine.

Author: Sunrisepeak

.github/workflows/validate.yml

@@ -191,7 +191,7 @@ env:
   # member on every platform. Measured: twelve workspace shards, all still cold
   # after fifty minutes. Batch descriptor changes that need a newer engine into
   # ONE pin move rather than several.
-  # ── 2026-09-11: raised 2026.9.6.3 -> 2026.9.10.2 ──────────────────────────
+  # ── 2026-09-11: raised 2026.9.6.3 -> 2026.9.7.1 ───────────────────────────
   #
   # Same shape as the move above, and for the same reason. `huxerui.huxerui`
   # is a Form A package whose build program (`mcpp/huxerui-build-rules`) calls
@@ -202,10 +202,25 @@ env:
   #     error: dependency 'huxerui': host module 'huxerui.rules' compile failed
   #     rules.cppm:274:61: error: 'package_name' is not a member of 'mcpp'
   #
-  # Verified at 2026.9.10.2: `mcpp test -p huxerui-module` builds the SDK from
-  # source as an index dependency and the member's assertion passes
-  # (`test result ok. 1 passed`), and the same member still fails on 2026.9.6.3
-  # with the error above. Nothing else in this PR needs the newer engine.
+  # 2026.9.7.1 is the FLOOR, not the current release, and the difference is
+  # load-bearing. 2026.9.10.2 was tried first and broke `mysql-connector-cpp`
+  # on linux and macOS: mcpp's scanner errors on a plain block comment. Reduced
+  # to four lines --
+  #
+  #     /*
+  #       module (exe)
+  #     */
+  #     int main() { return 0; }
+  #
+  # -- and bisected across releases:
+  #
+  #     2026.8.27.2  OK     2026.9.7.1   OK
+  #     2026.9.6.3   OK     2026.9.10.2  scanner errors
+  #
+  # so the regression lands after 2026.9.7.1, which is exactly the release that
+  # first carries `package_name()`. Taking the floor rather than the tip gets
+  # huxerui built without dragging that defect into every other member.
+  # Reported upstream; move to the tip once it is fixed.
   #
   # `index.toml` min_mcpp does NOT move, for the reason the previous entry
   # gives: the floor is about descriptor GRAMMAR, and `mcpp xpkg parse` accepts
@@ -215,7 +230,7 @@ env:
   #
   # The batching guidance above applies: this move pays the cold-cache cost, so
   # any other descriptor waiting on a newer engine should ride along.
-  MCPP_VERSION: "2026.9.10.2"
+  MCPP_VERSION: "2026.9.7.1"
 
 jobs:
   lint: