TICKET-034: Add test for alloc.__main__ module entry point

What's Wrong

alloc/__main__.py has zero test coverage. This module enables python -m alloc invocation and delegates to alloc.cli.main(). If the delegation breaks (import error, wrong function reference, exit code mismatch), the module entry point silently fails for users.

Evidence

• alloc/__main__.py lines 1–17: delegates to alloc.cli.main()
• grep -rn "__main__\|python -m alloc" tests/ → no matches
• tests/test_cli.py exists but only tests alloc.cli directly, not the __main__ delegation path
• The if __name__ == "__main__" guard on line 16 is never exercised by any test

Impact

• Users running python -m alloc could encounter uncaught import errors or wrong exit codes with no test safety net
• The __main__ module is the documented entry point in README.md ("python -m alloc.core --backtest") — if it breaks, the quick-start guide is broken
• Minor risk but high visibility: this is the first thing a new user tries

Suggestion

Add to tests/test_cli.py (or a new tests/test_main.py):