Support monorepo package globs in downstream tests

[ChrisRackauckas-Claude]

What changed and why

The reusable downstream workflow already defaults to Julia 1, but callers could only enumerate upstream package directories individually. This adds a trailing /* form to upstream-subdirs and develops the immediate child directories that contain a Project.toml.

OrdinaryDiffEq can use .,lib/* when it migrates its bespoke Julia-1.10 downstream job to this shared workflow. That addresses the ProbNumDiffEq resolver failure at the workflow layer requested in SciML/OrdinaryDiffEq.jl#4320.

Ignore this PR until it has been reviewed by @ChrisRackauckas.

Verification

Failing before the workflow change, with the new assertions applied:

julia +1.12 --startup-file=no test/runtests.jl
downstream.yml selects a monorepo package project | 12 pass, 2 fail, 14 total
missing: endswith(spec, "/*")
missing: readdir(dirname(spec); join=true)

Passing after:

julia +1.12 --startup-file=no test/runtests.jl
downstream.yml selects a monorepo package project | 13 pass, 13 total
EXIT=0

An isolated fixture containing a root project, lib/A, lib/B, and a non-package lib/not-a-package resolved to:

[".", "lib/A", "lib/B"]

Also passed:

actionlint .github/workflows/downstream.yml
typos .github/workflows/downstream.yml test/runtests.jl
git diff --check
julia +1.12 --project=<Runic environment> -e 'using Runic; exit(Runic.main(["--check", "--diff", "test/runtests.jl"]))'

Not verified

I did not run an end-to-end reusable-workflow invocation on GitHub Actions locally. The follow-up OrdinaryDiffEq migration should remain blocked on this prerequisite.