ci: probe the Actions runner before writing the Rust gate

gopher64-netplay has no runnable gate (#23), blocking !10 !11 !15 !18. The
build cannot run on the review box -- seven Rust worktrees took C: to 0.3%
free. Gitea 1.22.6's runner API endpoints 404, so runner availability, its
accepted labels, and its image contents are all unknown; this reports them
before the real SDL3/LLVM/submodule workflow is written against guesses.

.gitea/workflows/ci-probe.yml

@@ -0,0 +1,41 @@
+# Runner probe. Deliberately trivial.
+#
+# gopher64-netplay has no runnable gate on the review box (#23), which is why
+# !10 !11 !15 !18 are blocked -- a reviewer must not merge what it cannot
+# verify. Gating cargo locally is not an option either: each Rust worktree
+# materialises a ~10 GB target/, and seven of them took C: to 0.3% free on
+# 2026-08-25. The build has to move to the Actions runner.
+#
+# Before writing the real gate (SDL3 deps + LLVM + two submodules +
+# parallel-rdp is not a workflow to debug blind), this proves three unknowns
+# that the Gitea 1.22.6 API cannot answer -- its runner endpoints all 404:
+#   1. Does a runner actually pick up a job for this repo?
+#   2. What labels does it accept? `runs-on` below is the guess being tested.
+#   3. What is in its image -- rust/clang already present, or must the real
+#      gate install a toolchain?
+#
+# Delete once the real gate exists; it has no other purpose.
+name: CI probe
+
+on:
+  push:
+    branches: ["main"]
+  workflow_dispatch:
+
+jobs:
+  probe:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Who am I
+        run: |
+          echo "=== runner ==="
+          uname -a || true
+          head -3 /etc/os-release 2>/dev/null || true
+          echo "whoami: $(whoami)  home: $HOME"
+          echo "=== disk (the real constraint) ==="
+          df -h / 2>/dev/null || true
+          echo "=== toolchain already present? ==="
+          for t in cargo rustc clang cmake ninja pkg-config git curl; do
+            printf '%-12s %s\n' "$t" "$(command -v $t || echo MISSING)"
+          done
+          cargo --version 2>/dev/null || echo "no cargo on the image"