ci: refresh the published index before the workspace tests

The workspace jobs never refreshed the published index. The snapshot in play
is whatever the pinned mcpp release vendored -- the Download step `cp -a`s the
release's registry/ over ~/.mcpp/registry, on top of the restored cache -- so
it is by construction older than main, and it never moves, because the cache
is saved with that same stale copy inside it.

This stayed invisible because mcpp DOES refresh on a miss for a direct
dependency, and because every member so far resolved its packages either from
this checkout or from compat packages that have been in the index far longer
than any snapshot. tests/examples/godot-cpp-module is the first member to
depend on a package added in the same cycle THROUGH a Form-A package -- a
transitive dependency, which is the path with no refresh -- and it failed with

  error: dependency 'compat.godot-cpp': no package found ...
    index: local index 160c389 (never refreshed)

on all three platforms, minutes after `Publish Index Artifact` had already
republished an index that contained it. Reproduced locally against a snapshot
of the same age, and `mcpp index update` alone turns that run green:

  test result ok. 1 passed; 0 failed; finished in 72.94s

Author: speak-agent

.github/workflows/validate.yml

@@ -409,6 +409,30 @@ jobs:
           echo "MEMBERS=$sel" >> "$GITHUB_ENV"
           echo "selected members: ${sel:-<none>}"
 
+      # ── Refresh the PUBLISHED index before testing ────────────────────
+      # Most members resolve everything from this checkout, but a member that
+      # redirects a namespace other than `compat` gets the REST from the
+      # published index — and nothing here ever refreshed it. The snapshot in
+      # play is whatever the pinned mcpp release vendored (the Download step
+      # `cp -a`s the release's registry/ over ~/.mcpp/registry, on top of the
+      # restored cache), so it is by construction older than main, and it
+      # never moves: the cache is saved with that same stale copy inside it.
+      #
+      # mcpp does refresh on a miss for a DIRECT dependency, which is why this
+      # went unnoticed — the gap is a Form-A package's TRANSITIVE dependency.
+      # tests/examples/godot-cpp-module hit it head-on: the module package's
+      # own compat.godot-cpp dep resolved against a snapshot predating the
+      # commit that added it, and failed with `index: local index <sha> (never
+      # refreshed)` even though the artifact had already been republished.
+      # (Older members never noticed: their compat packages have been in the
+      # index far longer than any snapshot.)
+      - name: Refresh the published package index
+        shell: bash
+        env:
+          MCPP_INDEX_MIRROR: GLOBAL
+        run: |
+          "$MCPP" index update
+
       - name: mcpp test (workspace or affected members)
         shell: bash
         env: