Build the automated test matrix for precedence, includes, control flow, and cache correctness

[Matographo]

Summary

Create tests for input detection, variable precedence, rule precedence, include behavior, control-flow rendering, diagnostics, and cache invalidation.

Why

The project has a lot of precedence-sensitive behavior that must be locked down with automated coverage.

Acceptance Criteria

• Precedence rules are covered by automated tests
• Safe missing access is covered by tests
• Include cycles and cache invalidation are covered by tests

Dependencies

• Depends on Implement input auto-detection for string, file, directory, and stdin #4
• Depends on Implement deterministic rule precedence resolution #13
• Depends on Implement include processing with preprocessing and cycle detection #20
• Depends on Implement in-memory caching for parsed templates and include dependencies #24