Add tarball-test workflow for snapshot tarball CIruby_3_4

Port the daily snapshot tarball pipeline from ruby/actions into
ruby/ruby so the tarball build and per-OS tests run on every push and
pull request. The make-snapshot composite action gains a srcdir input so
the same logic can either clone ruby/ruby (daily upload from ruby/actions)
or operate on the working tree (this workflow).

ruby_3_4 keeps its branch-specific knobs in tarball-test.yml: ubuntu
runs with apt-mode=git-only and posts the ruby SHA notification on
schedule failures with branch-label=ruby_3_4, and windows uses the
legacy vcvars 14.2 path.

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

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