🧪 Add lfsr_tap test coverage

[perim]

🎯 What: The lfsr_tap function in dmath.cpp previously lacked any test coverage. It's a pure switch statement returning magic numbers for valid configurations.
📊 Coverage: A new test block in tests/test_dmath.cpp validates the exact output for boundary constraints (sizes 2 and 32). It also iterates over all valid input sizes (2 through 32) and checks that a non-zero tap is returned.
✨ Result: Test coverage for lfsr_tap is now significantly improved by exercising all supported configurations and explicitly verifying edge cases, protecting against regressions if the magic number generation is refactored.

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PR created automatically by Jules for task 7985626611225669783 started by @perim

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