233_bench_matrix: matrix.json's hub does not exist in the pinned tree, and the check only runs where submodules are initialised #599

Found while running the full e2e suite locally (332 passed, 1 failed, 35 skipped). Unrelated to the change that surfaced it.

What fails

FAIL: bench/matrix.json
  linux/gcc/mcpp-2026.8.11.3:   hub='modules/platform/src/platform.cppm' does not exist in the pinned tree
  linux/clang/mcpp-2026.8.11.3: hub='modules/platform/src/platform.cppm' does not exist in the pinned tree
  windows/clang/mcpp-2026.8.11.3: hub='modules/platform/src/platform.cppm' does not exist in the pinned tree

bench/projects/mcpp/mcpp-2026.8.11.3 is checked out at a749e9f (= v2026.8.11.3), and modules/platform/src/platform.cppm does not exist in that tree — the file lives at src/platform.cppm there. The path in matrix.json was written for the CURRENT layout after the module move.

The test's own message says what that costs: "every scenario that perturbs it would be reported skipped and the job would still pass".

Why CI is green

The check skips a workload whose tree has no mcpp.toml, which it reads as "submodule not initialised":

if not os.path.isfile(os.path.join(tree, "mcpp.toml")):
    ...
    uninit.add(c["project"])
    break

Only the bench workflow checks submodules out, so on every other job the three cells are skipped and the test passes. On a developer machine with submodules initialised, it fails — which is where it was found.

What a fix has to decide

Not just which path to write. The pinned tree is a historical mcpp, so a hub path that is correct today is wrong for it by construction, and the next module move breaks it again. The options are a per-project hub (the pinned tree's own path), or a check that resolves the hub against each tree rather than against one string.

Whichever is chosen, the uninit branch is worth revisiting in the same pass: a check that is silent on every CI job except one is a check whose failures are found by hand.