pkg: run conformance through ctest as the one canonical path

PROTO wired the suite into ctest (label "conformance": the end-to-end
`conformance` test that shells to run.sh, plus the native `conformance_cpp`);
the CI job called run.sh directly. Two entry points, and the required M0 gate
exercised only one of them, so the ctest registration could rot.

The conformance job now configures, builds velox_conformance_cpp, and runs
`ctest --preset dev -L conformance`. The dev test preset's
noTestsAction: error is the rot guard: an empty label match exits non-zero
instead of the old silent pass. build/sanitizers exclude the heavy e2e test
with -E '^conformance$' (the dedicated job owns that run; conformance_cpp
still runs under every sanitizer). ADR 0014 records the decision.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_0143aKiohmDiyefJBwHDJJqw

.github/BRANCH_PROTECTION.md

@@ -40,5 +40,7 @@ Their guards fail **loudly** (non-zero) once the lane is half-present — e.g. a
 filename is how a required check ends up green over nothing; the skip branch is only for
 a lane that is genuinely absent.
 
-`conformance` no longer has a skip branch. The suite (`tests/conformance/run.sh`) has
-landed, so the job runs it unconditionally and fails if the entrypoint is missing.
+`conformance` has no skip branch. It runs `ctest -L conformance` (see
+`docs/adr/0014-conformance-runs-through-ctest.md`); the `dev` test preset's
+`noTestsAction: error` fails the job if that label ever matches nothing, so a deleted or
+renamed registration goes red instead of passing vacuously.