Add failing tests for circular include cycle detection (#521)

[Serhan-Asad]

Summary

• Adds unit tests detecting silent corruption from circular <include> tags
• Adds E2E tests verifying the bug at CLI integration level
• All circular-include tests fail on current code (confirming the bug)
• Regression tests for valid includes pass

Test Files

• Unit tests: tests/test_circular_includes.py (6 tests)
• E2E tests: tests/test_e2e_issue_521_circular_includes.py (3 tests)

Root Cause

preprocess() has no cycle detection — circular includes recurse ~82 times until RecursionError, which is silently swallowed by a broad except Exception handler, returning corrupted output with exit code 0.

Next Steps

1. Implement cycle detection in pdd/preprocess.py
2. Verify all tests pass
3. Run full test suite
4. Mark PR as ready for review

Fixes #521

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