CI: build the Darling and Lite test projects in parallel jobs so each shard waits only for its own #5955

Claude posting for Erik Darling.

Leftover from #5949 (PR #5952). After #5952 the shards and build start when guard-build has built and uploaded the Darling and Lite test projects (357 s in run 38914724860). That job builds both projects in sequence on one Windows runner.

Proposal. Split guard-build into two jobs, one per test project, so each shard waits only for its own project's build: the Lite shards start when Lite.Tests is built, the Darling shards when Darling.Tests is. Expected gain: roughly half of the 357 s on each shard's path, for one more job's checkout and restore in runner minutes.

Scope. .github/workflows/build.yml only (guard-build, the needs: of the ten shards, linux, the whole-tree guards and guard-tests), plus the GuardStageWorkflowTests pins that describe the job shape. No new census or source-text test (#5833). Measure the same way as #5952: a throwaway draft PR run, before and after, shard start and run end.

Why not in #5952. The #5949 brief held everything else in build.yml still so the measurement isolated the Guard move. Unowned; a CI item, ranked with the other CI issues (#5942-#5947, #5950, #5951).