ci: run scan-build on pinned clang-18 + latest clang (informational)

ci: run scan-build on pinned clang-18 + latest clang (informational)

Split the scan-build workflow into two non-gating jobs, each uploading
its HTML report as an artifact:

- pinned-clang18: clang-18 / clang-tools-18 on ubuntu-24.04, so the
  checker set -- and thus the report -- is deterministic.
- informational-latest: whatever clang ubuntu-latest ships, to surface
  what newer analyzers see.

Both are informational (no --status-bugs): the tree still has known
clang-18 findings, so the run reports without blocking the build.  Once
the tree is at zero for clang-18, re-add --status-bugs to the pinned job
to turn it back into a gate.  Installs libpopt-dev so configure finds
popt under the scan-build compiler wrapper.