test(calculate): add unit tests for calculation utilities

[adcondev]

🎯 What: The testing gap addressed is the lack of unit tests for the internal/calculate package, specifically for DotsPerLine, MmToDots, and DotsToMm functions.
📊 Coverage: The tests now cover standard scenarios (e.g., 80mm/58mm paper at 203 DPI), zero values, negative inputs (where applicable), and edge cases like zero DPI.
✨ Result: The improvement in test coverage ensures the accuracy of these fundamental unit conversion utilities and prevents regressions.

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PR created automatically by Jules for task 1365553061994360617 started by @adcondev

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[github-actions]

👋 Thanks for opening this PR, @adcondev!

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• 👀 A maintainer will review your changes

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• ✅ All tests pass
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[github-actions]

⚡ Benchmark Results

📈 Performance Comparison

📊 Click to expand detailed results

Current Branch Results

BenchmarkNewDocument-4    	1000000000	         0.3118 ns/op	       0 B/op	       0 allocs/op
BenchmarkBuildSimple-4    	 3671428	       330.4 ns/op	     240 B/op	       4 allocs/op
BenchmarkBuildComplex-4   	  415224	      2721 ns/op	    1809 B/op	      25 allocs/op
BenchmarkToJSON-4         	  537505	      2056 ns/op	     592 B/op	       3 allocs/op
BenchmarkParseDocument_Minimal-4    	  490345	      2343 ns/op	     568 B/op	      14 allocs/op
BenchmarkParseDocument_Receipt-4    	   95006	     12509 ns/op	    2392 B/op	      34 allocs/op
BenchmarkCommandUnmarshal_Text-4    	  415986	      2704 ns/op	     568 B/op	      19 allocs/op
BenchmarkCommandUnmarshal_Table-4   	  280921	      4141 ns/op	     944 B/op	      21 allocs/op
BenchmarkTextCommandParsing-4       	  525998	      2112 ns/op	     544 B/op	      16 allocs/op
BenchmarkTableCommandParsing-4      	  246051	      4738 ns/op	    1000 B/op	      29 allocs/op
BenchmarkParseHexString-4           	 8791554	       135.1 ns/op	      48 B/op	       2 allocs/op
BenchmarkCleanHexString-4           	 6716810	       178.2 ns/op	      64 B/op	       2 allocs/op
BenchmarkContainsSequence-4         	368429950	         3.258 ns/op	       0 B/op	       0 allocs/op
BenchmarkCheckCriticalCommands-4    	31882032	        39.31 ns/op	       0 B/op	       0 allocs/op
BenchmarkDocument_Validate-4       	14421495	        83.30 ns/op	       0 B/op	       0 allocs/op
BenchmarkParseDocument_Simple-4    	  497493	      2392 ns/op	     568 B/op	      14 allocs/op
BenchmarkParseDocument_Complex-4   	  139507	      8474 ns/op	    1352 B/op	      26 allocs/op
BenchmarkPrintImage_Small-4             	     614	   1879542 ns/op	 3469937 B/op	      24 allocs/op
BenchmarkPrintImage_Medium-4            	     332	   3628723 ns/op	 5481096 B/op	      24 allocs/op
BenchmarkPrintImage_ThermalPreview-4    	     139	   8382857 ns/op	 5235268 B/op	  230723 allocs/op

Base Branch Results

BenchmarkNewDocument-4    	1000000000	         0.3120 ns/op	       0 B/op	       0 allocs/op
BenchmarkBuildSimple-4    	 3682395	       326.0 ns/op	     240 B/op	       4 allocs/op
BenchmarkBuildComplex-4   	  412598	      2708 ns/op	    1809 B/op	      25 allocs/op
BenchmarkToJSON-4         	  543272	      2066 ns/op	     592 B/op	       3 allocs/op
BenchmarkParseDocument_Minimal-4    	  512464	      2304 ns/op	     568 B/op	      14 allocs/op
BenchmarkParseDocument_Receipt-4    	   94244	     12614 ns/op	    2392 B/op	      34 allocs/op
BenchmarkCommandUnmarshal_Text-4    	  420345	      2743 ns/op	     568 B/op	      19 allocs/op
BenchmarkCommandUnmarshal_Table-4   	  279891	      4161 ns/op	     944 B/op	      21 allocs/op
BenchmarkTextCommandParsing-4       	  529851	      2111 ns/op	     544 B/op	      16 allocs/op
BenchmarkTableCommandParsing-4      	  244364	      4751 ns/op	    1000 B/op	      29 allocs/op
BenchmarkParseHexString-4           	 7226667	       160.9 ns/op	      48 B/op	       2 allocs/op
BenchmarkCleanHexString-4           	 6518689	       183.3 ns/op	      64 B/op	       2 allocs/op
BenchmarkContainsSequence-4         	295388701	         4.056 ns/op	       0 B/op	       0 allocs/op
BenchmarkCheckCriticalCommands-4    	32804752	        35.66 ns/op	       0 B/op	       0 allocs/op
BenchmarkDocument_Validate-4       	14523273	        83.48 ns/op	       0 B/op	       0 allocs/op
BenchmarkParseDocument_Simple-4    	  499491	      2381 ns/op	     568 B/op	      14 allocs/op
BenchmarkParseDocument_Complex-4   	  137887	      8469 ns/op	    1352 B/op	      26 allocs/op
BenchmarkPrintImage_Small-4             	     600	   1931674 ns/op	 3469939 B/op	      24 allocs/op
BenchmarkPrintImage_Medium-4            	     328	   3702212 ns/op	 5481087 B/op	      24 allocs/op
BenchmarkPrintImage_ThermalPreview-4    	     141	   8511979 ns/op	 5231178 B/op	  230423 allocs/op

💡 Note: Use benchstat for statistical comparison

🎯 Summary

• Total Benchmarks: 32
• Average Speed: 2565240 ns/op
• Average Memory: 2396936 B/op
• Average Allocations: 41093 allocs/op