refactor: extract output funcs to shared lib

[gigatim]

Summary

Standardizes shared infrastructure across the 4 gigalixir buildpack repos (elixir, mix, phoenix-static, releases).

Output functions (lib/output_funcs.sh)

• Extracts output_section, output_line, output_warning, output_stderr, and output_indent into a shared lib/output_funcs.sh
• Replaces inline definitions and legacy function names (info, indent, header) with the standardized versions
• The shared file is identical across all 4 repos

Test framework (test/test_framework.sh)

• Extracts common test plumbing (cleanup/trap, output stubs, test()/suite()/reset_test() helpers) into a shared test/test_framework.sh
• Each repo's test/.test_support.sh is now a thin wrapper that sets repo-specific variables and sources the framework
• The framework file is identical across all 4 repos

Makefile

• Adds a Makefile with a default test target that runs all executable test files
• Updates CI workflow to use make test

Test plan

• All tests pass locally via make test in each repo
• CI passes on all 4 PRs