experiment: switch to @main floating ref to test hypothesis 2 (SHA resolution)

If @main parses where @cc5a372a does not, the bug is in GH's SHA
resolution for cross-repo reusable workflows specifically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/rust-ci-v2.yml

@@ -14,4 +14,4 @@ permissions:
 
 jobs:
   rust-ci:
-    uses: hyperpolymath/standards/.github/workflows/rust-ci-reusable.yml@cc5a372af1af1b202c17f1b21efd954e6c038bef
+    uses: hyperpolymath/standards/.github/workflows/rust-ci-reusable.yml@main