ci: validate every member with mcpp 2026.9.29.4; latest_mcpp -> 2026.9.29.4

2026.9.29.4 (mcpp-community/mcpp#741) links a program a workspace member ships
through artifacts with its own closure's line and expands ${mcpp.bin_dir} to
the declaring member's product directory.

Author: Sunrisepeak

.github/workflows/validate.yml

@@ -397,7 +397,12 @@ env:
   # 2026.9.29.3 (mcpp-community/mcpp#740): a program a member ships through
   # `artifacts` waits for no plan-level runtime file, and its own runtime
   # files are placed beside it in the member's product directory.
-  MCPP_VERSION: "2026.9.29.3"
+  #
+  # 2026.9.29.4 (mcpp-community/mcpp#741): a program a member ships through
+  # `artifacts` links with its own closure's line, and `${mcpp.bin_dir}` in an
+  # action a member's build program declares is that member's product
+  # directory.
+  MCPP_VERSION: "2026.9.29.4"
 
 jobs:
   lint:

index.toml

@@ -134,7 +134,12 @@
 # root; 2026.9.29.2 is the version the full sweep ran with, and 2026.9.29.3
 # adds the runtime placement of programs shipped through `artifacts`. None of
 # the three changes descriptor grammar.
+# ── 2026-09-29: latest_mcpp -> 2026.9.29.4, min_mcpp unchanged ─────────────
+#
+# Moved in the pull request that moves validate.yml's pin to 2026.9.29.4,
+# which runs every member on every platform with it. The release changes no
+# descriptor grammar.
 [index]
 spec        = "1"
 min_mcpp    = "2026.9.18.3"
-latest_mcpp = "2026.9.29.3"
+latest_mcpp = "2026.9.29.4"